Can i swap my iphone 4 sim with an iphone 4s?

I have an iphone 4 now with sim card and a plan for it. Now if i buy a new iphone 4s will all i have to do is put the iphone 4 sim card in the 4s and everything will be fine? Will i have to talk to my service provider?

You don't have to call anyone. Just put the old sim card into the new phone. Everything will work out.

It will work if the new phone is from the same provider or unlocked