Flag, flag link, and flagging: create an advanced relationship
Using Drupal rules module you can set up the right triggering conditions. It is possible to use rules to alter the data in fields arbitrarily, however your question is a little confusing because bands are also described as users, so you would need to make use of user related fields in the rules configuration. Take a look at the large rules documentation section and this intro to the rules module to get an idea. Flag module is compatible with rules.