If I subscribe for extra storage, what happens if I ever cancel that subscription? Can I still get to everything in my library but just not add anything more?
1) storage subscriptions apply to files you store/sync online, not local files. You will always be able to add more files locally and Zotero never deletes or removes access to local files. (Or, in other words, you can have and sync a library of almost any size for free as long as you don't care about syncing the file attachments)
2) for the online files, Zotero gives you a grace period during which you will be able to access but not add to storage and then removes files (I believe in the order in which they were added) from the online storage until you're back down to you free storage. Again, this only removes the files from the server. You still have access through your local copy of Zotero.
Upgrade Storage
2) for the online files, Zotero gives you a grace period during which you will be able to access but not add to storage and then removes files (I believe in the order in which they were added) from the online storage until you're back down to you free storage. Again, this only removes the files from the server. You still have access through your local copy of Zotero.