to unscrew the nuts to release the base from the floor. The water, condensation, and waste on the metal will cause it to weld together.
So, the best way is to get a hax saw and carefully saw the bolt in half and that way the base will come off and you can then remove the old wax ring and the colar and install new bolts and wax ring. Then be so carefully about installing the new bolts. never use a socket set and ratchet down on the bolts.
You run the nut down snug to the base, then sit on the comode and waller around a bit to cause the wax ring to compress, then with a cresent wrench tighten down the nuts again snug, then repeat the waller method and tighten down again. If you don't do this the base may crack and you will be in big trouble.