This would be messy, and add quite a lot of UI bloat.

So documented a workaround instead, that works equally as easily as a new inbuilt UI resolver would:

"You cannot change field data types after you have created them (except between field types that have the same "storage" type) as this would affect the integrity of any data that may have already been entered into them. A workaround is to export to CSV, delete the field, create a new field with the same name and the new type, and reimport the CSV."

Essentially it means we switch creating our own resolution interface, to suggesting people do it themselves in Excel.