I’ve had to do this a few times
What I did was create a new field in the datatype that was a list (as far as I’m aware you can’t modify a field structure, only the name after it’s created).
Then on page I placed a button (I have a random page I use for things like this).
I made a backend workflow to ‘make changes to a list of things’. Say it’s a field on your user datatype you’d like to bulk change from the old field to new field. Then for the list you do a search for users and have no constraints (so it brings back all users - add constraints if you don’t want all of a thing), then add your new list field as the field to change by adding ‘this user’s old field type’.
Link the workflow to the button on the page, preview the page and then click the button. VOILA!
If you need it to happen to your live database, then if it worked in dev push to live and repeat