You are loading an entirely new actor, It doesn't know anything about the old actor. So simply remember the position of the old one (in some variables) and after calling LoadActor set the position of the new actor.
Also, don't forget to unload the old actor first! Or it will stay loaded in memory.
As for scrolling, you need to tell the game which actor should be tracked by the "camera".
Game.MainObject = newActor;
I'm afraid the change will never be *instant* though, loading the actor takes some time. Various games solve this problem with some tricks, such as the character walks to some hidden place and gets back in a new dress (i.e. the new model is loaded while the character is hidden).