I was in the same situation, i played time ago with a team, and after months without play, my team don't exists. That's because all the players are older, and... just leave your team...
or, if you are not playing, and you don't renew the players contract, they just leave your team.
That's all..

so, you don't loose levels because that's not possible, just that, your old players leave team for the age, and others for the no-renew cotract, if you don't do that they are free to go to another club.