If anyone is coming on this post from now, there is a mod you can download from Apps named 'account_update_tax_tags' which allows updating previous VAT return to what it was before Odoo changed the tax mapping (i.e. v18 to v19).
First, you need to re-configure your taxes and then you go to Accounting -> Settings and clic on the button named by something like "Update tax tags on existing accounting entries"