There isn’t any good way to update field values on all posts when you change field settings.
It is possible if you write PHP. Do a WP_Query() to get all the posts, loop through the posts, get the field, check the values, remove the values for the choices you’ve removed and then update the field if needed.
If the value must be removed on the front end then I would probably build a check where the field was used in the template rather than try to update all of the posts.
