Vault Basic to Pro Updating Category
There isn't anything that will go through all of your files and place them into categories automatically.
However, if you have category rules created, you can select a group of files, select Change Category and in the Select a new category drop-down, select "Apply assessment rules".
Multiselect across separate folders works because the query/search results lists all files regardless of where they are. Considering, of course, that you are searching all of Vault (or a top folder containing all the content).
I would recommend building a search with the Find tool, saving it, and then launching your search from the "Saved Searches" section of your Vault browser (left pane). This way you can easily maximize the results screen and click on More Results until you have them all. But you can also do it in sections, since there is no harm in changing a chunk at a time.
Your search criteria can be built in many ways, and I believe Vault is flexible enough to narrow this down as much as you need it. All you have to do is know what unique features/flags/properties those Vault Basic files have. Even if it is just their creation date being up to a certain date.
Other useful properties to work with as a filter for your situation may be "State", "Lifecycle Definition", "Date Version Created", "Date File Created", "Created By", "Original Create Date", "Folder Path", and/or "File Extension".
