1. If you have assigned a bank to a property and started with your accounting for that property the system will stop you from another bank assignment, or change, for that property. 
  2. Additionally, no bank deletion is possible.  This is done to keep your accounting accounts in balance according to double entry accounting principles.

Many of our users are not accountants nor have a formal accounting background and we save these folks the pain to learn the debits and credits of each account type. The bank management must be managed by the system to enable this simplification.