So last week I bought KRZ and started playing in advance of Act V...Now that Act V is out, I have downloaded the updated .zip file, but how can I retain my save data from Acts I-IV? Or does it not matter as long as I'm not in the middle of an Act? 

Nevermind, seems like it worked fine on its own. I extracted it to a separate folder from where I have the previous version of KRZ. It could even tell that I had just finished Un Pueblo de Nada even though I ran it from my previous KRZ folder.