how to recategorize / merge old autofill categories

If you are using current QMac Subscription version, then you will want to open the Categories window (menu Window > Categories). Make a backup before you start.

In this window, you'll see a Merge Categories button at the bottom. To use this, select two categories (using click for the first one and command-click on the second one). The categories must be at the same level of the hierarchy. You'll see the Merge Categories button become active. When you click it, you will be asked which name you want to retain.

If the categories you want to merge are not at the same level, you can drag and drop to put them at the same level. For example, you can drag and drop a subcategory up to become a top level category. If there's already a category of the same name, you'll be asked how you want to merge them.

If you just need to rename a category, select it and click the pencil icon at the bottom.

If you make a mistake, each of these actions can be undone in reverse order via Edit > Undo (Command-Z) as long as you don't close the Quicken file,

You cannot hide categories in QMac so they don't show up in the Category dropdown menu when entering a transaction (although this has been requested). A suggested workaround is to rename any categories you no longer actively use to start with "z" so they fall to the bottom of the list.

When I say "menu Window > Categories", what I mean is that you click the Window option on the top menu bar. After clicking Window, you will see an option "Categories". Select that.