The suggestion is to add a new functionality which "suggests" the prepayment invoice/ prepayment credit memo instead of posting these documents directly (create unposted prepayment documents and allow modifications before posting).

This would give users the option to change the document before posting, as it is done for other document types.

Usually a user would not change the prepayment invoice.
But adding this flexibility would allow needed changes, for example when the VAT rates get changed.