Looks like it's comparing strings, not numbers. If you want to do a one-time update to increase the number of digits, what about
UPDATE Invoices SET InvoiceID = MID(InvoiceID, 1, InStrRev(InvoiceID, "-")) & RIGHT("00000000" & MID(InvoiceID, InStrRev(InvoiceID, "-") + 1), 5)
where the last...