Hi welcome to the board.
I'm planning the same sort of project but i'm taking seats from a 5 series and sticking em in a 3 series. The front seats might fit straight away, if not change the cradle underneath for the ones that are origanally in your car, and as for the back, I don't know if yours will straight swap or not, but wot I am doing is removing the leather and using it to recover my back seat as the new one wont fit in, that way I get the same leather and same style all the way through.
Don't know if this of any help to you but I can't answer yes or no as to wether the new seats will just sraight swap...sorry mate but I hope it gives you ideas on changing them at least.