I have a CDL license, which was issued before 1999. It is ok to keep it? Because i'm not part of any federal operation, by the way.
There's no problem with that. you can keep it alright. In fact,you are elegible for Excepted Intrastate. For that, you must check you license has the required restrictions.
What about if it was downgraded because medical status?
If that's the case, you'll need to visit a DMV office near you to pass the process for upgrading back your license to a CDL.