John,Good to hear you're enjoying the World Cup and still getting your playing in.
Your questions:
1. Combs should theoretically outlast the player. If the comb gets broken or if it is made of wood and warps, then of course you will need to replace it (buy a new one).
2. I suppose it depends on the brand. I do know that with the Lee Oskar models, all the combs are identical except for the lowest keys like "G" (actually they may be the same also, but the cover plates are definitely deeper to accomodate the vibration of the longer reeds of lower keys). The comb for a key of "C" or "D" will be the same as long as you don't have a problem with the key stamped on the comb being different than the actual key it is in. If you put "C" plates on a "D" comb, it is now a "C" harmonica regardless of what is stamped on it.
Play on,
Dave Gage