I was wondering about the feasibility of getting messages anytime an upvoted answer has been changed. This would help if there is a substantial change in the answer, whether you would still want to keep it upvoted.
If you star a post, then all edits to the post and its answers will be visible in your "favorites" tab (the tab gets an indicator on it whenever there are any changes)
Regarding inbox notifications, that has been proposed many times (example, another one) on the main meta, for various similar features. It's been declined most of the time, since it can get too noisy for active users.