One thing that I am kind of amused by is the fact that my bank changed their system for numbering accounts after I joined initially. So the account number that I started with (and which was printed on my checks) is not my account number any more. For whatever reason, my bank still accepts those checks that have the wrong account number, even though I know for a fact that they won't accept that account number in any other form (I had to go in and update my direct deposit at work when they made the switch, because my employer wasn't able to use the old account number to deposit my paychecks anymore).
Back end processing has ways of linking old format account numbers to new format account numbers. It's annoying for those of us in deposit operations, and it costs the bank some amount of money. But it's less annoying than having a whole bunch of upset customers who are forced to buy new checks or spending money giving all affected customers new checks. There will likely come a time in the future when the bank decides that enough time has gone by and there are few enough customers affected that they decide to stop honoring the old checks.
On that note, my employer recently went through a major merger. They tried to keep the hassle to a minimum for the customers of the bank that got swallowed up, but some hassles were unavoidable. One of the issues was line of credit (lca) super checks. Those checks are a lot more difficult to process back end than checking/savings account checks (known as demand deposit accounts, or dda's). At first we kept taking the old bank checks, but sent all affected customers multiple letters letting them know that their account numbers would be changing and that as of a particular date, we would no longer be accepting the old bank checks. As that date drew nearer, we sent out additional letters and in one of the mailings we included 3 starter checks to hold them over until they could order new drafts. It's worth noting that this particular bank doesn't charge for lca drafts, at least for the home equity accounts. The next step was that when the date finally arrived, the tellers were instructed to discontinue accepting old bank drafts, but offer the customer an alternate method of taking out the advance, then get new drafts ordered before the customer left the store. If the old bank drafts came in through the back end (from a merchant or another bank) we would still process them, but send out a reminder letter to the customer with some additional starter checks included, advising them to get new checks ordered, as even that service would be discontinued past X date. X date arrived. We started returning the checks to the payees as "invalid account number". After all those warnings and starter checks, we still
had customers write old bank checks. These got returned, but we did not charge a bounced check fee. We again sent a reminder letter with even more sample checks, and if the customer complained, we would even pay them back for any provable fees charged by the merchant up to Y amount per item. Customers still complained that we were being unreasonable after at least a dozen warning letters and up to three mailings of starter drafts over the course of a little over a year.