How do I switch ERP while my shop keeps running?

Your shop runs on Shopify, Shopware or WooCommerce, not in the ERP. What can go down is the path from order to parcel: if an order does not arrive in the new system, it is not picked, invoiced or booked. If stock levels in the ERP are wrong, the shop shows wrong quantities.

Nobody can guarantee anything. But for each phase you can define how you recognize it as done. All recommendations on this page are our own practical rules, not a requirement from weclapp or authorities; sources are given where one applies.

Still unsure whether you need an ERP at all? Settle the whether first: Invoicing tool or ERP? The overview is in ERP for online merchants.

List what runs today: shop, marketplaces, shipping with labels, payment, returns, accounting and the undocumented manual work. Decide which system holds which data. Ask your tax advisor which figures they need on the cutover date (such as open-items lists and stock values) and which DATEV interface they use. The interfaces are listed on the DATEV marketplace; the sources are at the bottom of this page.

Test first with a test or development shop and a few test items, not with the live shop. According to the weclapp help center, for Shopify: items flow in both directions, customers and orders come from Shopify to weclapp, weclapp sends stock only to the shop, every 5 minutes after activation. Prerequisites are the same product ID, variant ID and SKU in both systems, plus assigned warehouses. Reconciliation with marketplaces is explained in Keeping stock and orders in sync between shop and marketplaces.

Which connection is in which package is shown on the pricing page: according to the feature table, online shops are in the Dienstleistung (services) and Handel (retail) packages, marketplaces only in the Handel package; the package overview lists shop integration under the Handel package. According to its listing, the Shopify app is free only with an ERP Handel license; additional charges may apply. Clarify the package and the cost of the connection before signing the contract.

Run test orders through to the invoice: the normal case with label and tracking, special price, partial delivery, return with credit note, cancellation, international, every payment method. The test is passed when every case runs through to the document, amounts and stock levels are correct and the tracking arrives in the shop.

Now the master data goes into the ERP: items, customers and suppliers. According to the weclapp help center, the import wizard works with CSV templates for import types such as customers, suppliers, sales items and item prices. Clean up duplicates in the import file, not in the old system; that way the old data stays unchanged. How you link existing shop items to the ERP items is something you work out in the test, before you connect the live shop.

You do not import warehouse stock through the wizard but through the cutover-date stocktake. According to the weclapp help center, you load it via a CSV template, the warehouse is locked while you do so, a valuation price is mandatory, and the booking prices are fixed. Ask your tax advisor in advance what value the opening stock should be valued at, and try the process with a partial stock. This does not replace tax advice.

The phase is done when spot checks of items and customers match the old system and the trial stocktake runs without errors.

In parallel operation you work productively in the old system while the ERP runs alongside with test orders or copies of real orders. At any point in time exactly one system performs each task: fetching orders, reporting stock, sending documents, reporting tracking back. Otherwise you get duplicate orders or two invoices. The pricing page lists automated document sending; clarify in advance whether you can control it in the ERP, and send only from the old system until the cutover date.

On the cutover date you work through this sequence:

1. Close out open deliveries in the old system and check reservations in the ERP.
2. Carry out and book the cutover-date stocktake in the ERP.
3. Create the real number range for invoices.
4. Connect the live shop and spot-check the item mapping.
5. Activate order transfer, and beforehand set the "Orders from" date in the Shopify settings to the cutover date, so that neither old orders are duplicated nor new orders missed. Then activate stock transfer. From now on the ERP stock determines what the shop shows.
6. Hand over the remaining tasks one after another and watch the abort criteria.

Invoices from the old system either stay open there or are carried over into the ERP as open items. Decide this, and how to handle returns on old orders, in advance with your tax advisor. This does not replace tax advice.

Before the cutover date, define how you recognize an abort and what happens then. Examples: an order is missing in the ERP, the number of orders in shop and ERP differs at the end of the day, a label cannot be printed. Then you switch back to the old system.

For the fallback the old system must remain accessible; only cancel it once the abort criteria no longer apply. Independently of that: anyone who keeps records subject to retention on data carriers must, under § 147 (2) AO (German Fiscal Code), keep them available at all times during the retention period, be able to make them readable without delay and be able to analyze them by machine. For a system change, the GoBD (BMF letter of 28.11.2019, last amended 14.07.2025) set out in margin numbers 142 to 144 the conditions under which the old system does not have to be kept, including a transfer of the data into a new system or archive that is equivalent in quantity and quality. This is for orientation. Which export is sufficient for you is something you clarify with your tax advisor before shutting down. This does not replace tax advice.

Under § 14 (4) no. 4 UStG (German VAT Act), an invoice must carry a sequential number with one or more series of digits, assigned once by the invoice issuer. You should therefore avoid duplicate invoice numbers. According to the weclapp help center, you set up number ranges with a prefix and last value, optionally with a validity period; they are meant to produce unique numbers. Test invoices with their own test prefix so that no test number ends up in the real range. You create the real range on the cutover date, with a new prefix or continuing from the last invoice of the old system. Whether you continue the old system's count and which variant suits your accounting is something you clarify with your tax advisor. This does not replace tax advice.

At a glance: what happens in each phase, how you recognize its end and where it typically gets stuck.

Count backwards from the cutover date: parallel operation comes before it, so a cutover date on 1 January means parallel operation falls right in the middle of Black Friday and the Christmas season. If you have an autumn peak, you are better off choosing a quarter start after it, for example 1 April. Without a peak, the start of the year is the obvious choice because accounting periods then begin cleanly in the new system. Agree the date with your tax advisor. This does not replace tax advice.

A reliable time estimate only exists after the inventory. According to the pricing page, weclapp's free trial lasts 30 days and ends automatically. For data migration and parallel operation it may be too short; plan from when you need a paid tenant. According to the pricing page, monthly licenses can be cancelled with one month's notice, longer terms only at the end of the term, which matters for your fallback plan. Ask in advance whether connections are possible in the trial and what happens to the test data afterwards.

A fictional merchant with 2,500 orders per month (assumption) sells via Shopify and two marketplaces; their orders run in an order tool, supplemented by several apps. They switch at the start of a quarter after their peak, test in the development shop and include returns, which so far run by hand, in the test plan. Before the cutover date they define who does what: