I stayed 223 on one and the other 6xc was a new action so nothing there to replace on that one.
If youre wanting to switch it up a 223 has a .384" bolt face. Means you can go 223ai, 204, 222, 6x45, 300bo, 20 prac and a few more but those are the main ones. Some have factory ammo options, some dont.
If you wanted something else thats larger you would need to have the bolt face opened up or you need to get another bolt for it, ptg or something like that. I would get a new one piece bolt before I opened it up. Those arent home smith jobs though like a remage is though.