I had many problems with betas, but hey - who upgrade program without made backup of old database? You can do it by copying files or using internal function to create backup. You can even copy everything (including program itself from “Program Files x86”) and revert back using just copy function. I made it lot of times using earlier versions of beta. It’s obvious that new version will have new database format that must handle new functions.
It’s a shame that eM Client 8 was released soo fast, probably way too early, but in my case all major problems related to emails/crashes was resolved and now I’m using stable version on three computers without any problem. Maybe because I was active during beta testing?