yes you can install it and it won't affect any of your previously saved games. and yes, it will cause a very, very big change in your game:
it will make it a whole lot better. <img src="/ubbthreads/images/graemlins/up.gif" alt="" /> it will add new prestige classes, which are awesome by the way. it will open up new feats & skills, a lot of them epic ones. you can play the oc & sou with the new prestige classes, assuming you meet all the requirements. but they shine best when played in hotu. you'll go crazy figuring out how to multi-class your characters since you can pick two prestige classes without suffering any exp penalty, iirc. you will really feel your character grow in power, have better control over your henchmen, and henchmenn AI is better. in short, you can kiss your social life goodbye, assuming you have one right now. lol.
it's really one of the expansions that is worthy of being called an expansion.
p.s. good luck in charwood. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
Drink Up Ye Cider. 
