1. Can Bitcoin change to POS if wanted to by the community?
2. Is POS really an advantage (if it is - is it an advantage because it saves energy, or because it diffuses power)?
1. Yes it can be changed to POS but it is quite hard to reach any consensus this days for bitcoin, look at current block size debate.
2. I don't think it has any advantage, POS is only good for whales and they will keep dumping whatever they earn as interest.