In the case of Address fields, you will find there is some extra complexity to it, because there are several places in the code that reference these fields (pull them together to show a complete address, or in a feature that copies address from Account to Contact, etc).
So you’ll have to see how to handle this.
Are you ok with editing PHP files? When you can’t get Studio to do what you need, that is the way to go: https://docs.suitecrm.com/developer/metadata/
I see two ways of trying this:
just try to make that field a dropdown in the Edit view, editing metadata if necessary
make a new field that is a Dropdown. Hide the old field in the views. Then add a Workflow that, whenever the new field is changed, copies the value into the old field. This ensures that those other features I mentioned keep working.