You cannot change an invoice number on a paid bill
Only the following three fields may be changed on a paid bill:
- 1099 Transaction
An Invoice Number is deemed an internal control so it can't be edited on paid bills. You can, however, create a new bill, add the invoice number to that bill, add a comment pointing back to the original bill and check, enter 0.00 as the amount and use the Assign Check Number button to assign a fake check number.