I mean it’s still do-able. Just make the new field, but this time check the box to make it a list. Then run a backend API workflow on every User. You would do “a” for the parameter, with the type User. then for the first action do “Make changes to a thing” (type is User) and you’ll be changing “a”, and modify your newly created field that is a list of Departments, and do “add a’s Department”.
In your app now you’ll have to go around anywhere you have referenced the single field Department and change it to a repeating group with the data source to the User’s Departments (the list version). Any dropdowns you have to pick departments or anything would also need to be changed to accommodate working with a list now.
If you delete the single field of Department, you can go to settings and optimize the app, and just check only the deleted field Department. Once it deletes the field it will throw any errors that you need to correct that are referencing the single Department field