VAT registration is backdated - how to edit all invoices


I registered for VAT a few months later than I should have.
I now need to change all sales and purchase invoices to include VAT.
I have a problem with editing sales invoices - I don't know if I'm doing something wrong.
Since the invoices have already been paid and allocated with payments, I unallocated them first.
However, Sage won't let me change them.
Is it possible to do a journal to credit sales and debit VAT (for sales invoices)

Thank you