I used Classic Shell on 8.1 OK but haven't bothered since reinstalling it on my SSD, because I still use W7 as my main OS and I'm just leaving W8.1 + Update 1 idle waiting for Updates 2 and 3? which are reputed to reinstate a proper Start Button.
Till then I'm leaving it vanilla to avoid complication, but afaik you should have no problem with CS on 8.1 though you might have to reinstall it after the upgrade. (Don't forget Update 1)