Hi Geoffrey,
Unfortunately List - User fields are one of two specific field types (the other is Multi-Select Text) that once a field is created as that type or converted into that type they aren't able to convert back. They are fields that format differently then most fields in Quick Base in order to hold multiple values. Once they are applied to that type there isn't a built in function to convert the existing field. The fix is to create a new field to mirror the data in the right field type (in this instance a user field) and then once that data is in place replace the existing field. I'm sorry there isn't a technical fix I could recommend in this instance.
