First off, never use a slot as your "staging", or "development" environment. A slot utilizes the resources from the primary web app so not really a good idea to have your "development" site utilizing the same resources as your production site. I know I'll get some pushback on this but slots should really only be used for deployment of new code and getting your site warmed up prior to swapping slots out.
To answer your question, typically what we do for upgrades and hotfixes is to run test of the upgrade on a backup of production. Get all the kinks worked out then run it again on production later when we're ready to go live. By that time you'll have the procedure figured out and hopefully all the gotcha's worked out.
This typically means you'd take a backup of the database on production when you're ready to run the upgrade. There may be some time, depending on how many upgrades you're running and amount of traffic on your site, where you may miss some data (form submissions, contact's activity tracking, etc.) but that should be pretty minor all things considering. Run the upgrade on your copy of the database, then swap the slots out.