If it has to be done Just be careful not to touch the beer with your hands although maybe you can drain some of first if its a problem. maybe some lube might help it seal a bit better. ky will do it. Is it a king keg? they seem to have problems with he tap . If it is the lid maybe a new valve is needed.
No problem opening the keg to re-seal the lid. If you have primed then you can just re-prime afterwards. As rossa said a little ky or vaseline or something will help seal the o-ring in the lid. BUT it will kill your head retention if it comes into contact with the beer, so be sure not to tilt the keg over.
