This happened to me recently as well. An update to my bank's app required a slightly higher Android version, it went from 2.2.x.x to 2.3.x.x. So I finally installed CM7 on my Motorola, and the bank app works perfectly again, and the phone is much better than it used to be overall. See what the new requirements are for your bank's app now and see if it's possible to upgrade your phone's ROM to still use it.
That's the only way to do it. I seriously doubt complaining to your bank will get them to make it compatible again with lower versions.