If you flip the skeleton it'll all go horribly wrong. Animation mirroring is tricky - to do it properly you need to do a load of matrix maths so it's not something to be taken lightly. You may be able to cheat by flipping the rig in max and constraining a new rig to it. It'd probably help if you posted a picture of the…
thanks for answers guys! yes, I would like to use it for mod the game. the question is always bothers me and I never had a strong solution for it. the rig is not CAT/Biped, it's just a bones and most of the time is just "helpers" instead of real bones
It'll probably render OK in max but it's highly likely it'll explode if you try to put it in a game. There are ways to do what you want without rebuilding the rig and reskinning manually but it's one of those situations where if you have to ask how you probably shouldn't try it.