The upgrade should create a copy of your old catalog along your LR 5 catalog and the old catalog should be intact. You should be able to open the old one with LR 5 just fine. Did you install LR 6 as standalone or from creative cloud? The not being able to export sounds like you should log out of creative cloud and back in. If it is a standalone version I'd check the permissions on the folder you are trying to export too.