1. You can change the trigger by deleting the original trigger and then simply adding a different one. Complicating this procedure is the fact that you can't delete the trigger if any other components are using fields from it. So, if you had an email action that used fields from the trigger, you'd have to remove those fields from the email before getting rid of the trigger.
2. There is no trigger to start a flow from modifying a particular field of a list. The old way of doing this is to have an extra field to store the older value, and then compare them. For example, if your field is "status", create a new field called "status_old". The flow would fire for every list change, and would compare those values. If the values were different, then it should proceed on, doing whatever it needed to do, and then update status_old with the current value in status.
