There is a reason the call it thermoplastic.. By definition it can be remelted and reused. Most plastics can be reground and reprocessed several times without degrading them.. . Most process in excess of 300 degrees, so a little warming in hot water isn't going to hurt them.. Dunking in cold water won't either but really only speeds the cooling process,, You can let them air cool if you want.. Definitely doesn't stress them as much as sand paper or a file, where you are actually removing material and introducing stress risers with machining marks..
Ultra violet will degrade them in time, but most new arrows being shot regularly won't live that long.. It's a good idea to renock any old arrows you get before shooting them, as most are degraded and will fail on the string