I've been using Q Business and Personal and am about to downgrade to Q Classic Deluxe. Will my Q data file be readable by Q Classic Deluxe?
Yes. You just won't see any of the business information.
Actually, any "business" type accounts that you have will just become regular Asset and Liability accounts in Deluxe … but you'll lose all of the business specific reports and functions.
You won't lose any data.