My suggestion is to contact USAC and see if they have your license on file. That way you control where you want to race - to an extent. Starting over as a Cat 5 may preclude you from racing with the guy who is encouraging you to come back. You have to do 10 qualifying mass start races before upgrading to a Cat 4. Theoretically, masters racing does not allow Cat 5 racers. So it is a crapshoot if there will be a 55+ group that a 55+ cat 5 racer can race with. It seems to be less of an issue with time trials versus mass start racing.
And you can always downgrade to a Cat 4. I am not sure why you would do that other than to race is the limited number of 55+ 4/5 races that may be offered.