Hi @scipio
Changing from one method to the other requires you to make manual adjustments with your first return as Cash locks the payments into a vat return and accrual locks the invoices. So when you are creating your first return using the new method nothing else has been locked previously.
There are a few posts on this topic already on the forum: Changing Vat from Cash to accrual
Cash to Accrual VAT
Change to Accrual
You may find these threads will help you