I have series named like INV-.#####, after creating many invoices, I have to update those to 6 # and not only 5, I changed the naming series to that value, updated it, but the old invoices didn’t update, they kept their old naming. While new invoices use the new naming rule.
If you make manual changes to the invoice numbers, it will damage referential integrity - not recommended!
You could do this via SQL commands, but the risk of corrupting your system is massive