The point of true up-licensing is that if you had more users than your license allowed, you need to pay for those users the next time you renew your license. What you want to do sounds like circumventing that, you shouldn’t try to.
As resetting that count can basically only be used to circumventing the license model I don’t think it is possible.
All that said, I’m not pleased with the fact that (as far as I can tell) there’s no way to ever go to a license with fewer seats than you have now. Luckily the company I’m working (and maintaining GitLab) for is growing, so we don’t really hit that.
Blocked users doesn’t count, so if you just block users when they “die”, you shouldn’t hit this issue. If you have failed to, all I can suggest is that you contact you account manager (or whatever it is GitLab calls it) and plead your case.