we have seen other characters go from security/tactical to operations and vice versa including worf,sito, and obrien so I would say its realistic
No one is saying that it's not realistic going from one of those departments to another. What we're mostly saying is that... Well, to be blunt, Kim is pretty useless and is the last person that should be head of security. He certainly wouldn't make me feel secure.
Would you want Harry Kim guarding you from assassination?
Absolutely not. But Janeway... especially if we got some cuddle time... Oh yeah.