Lookup pulls string, not ID

Seems like a lot of questions along this vein go the other way -- people want to pull a string not the ID. I actually have the opposite problem: I have lookup fields in forms that date back to 2013 that pull strings, when what I need is IDs.

Here's the Form definition text in question:

        must have School_Group
        (
            displayname = "School/Group"
            type = picklist
            values = Schools.Name_of_School
            displayformat = [ Name_of_School + " - " + City ]
            allow new entries = true
            [
                displayname = "Add a School or Group"
                alignment = bottom
            ]
            sortorder = ascending
            searchable = true
            width = 206px
        )

We've now reached a level of integration with it that I want to pull the value of that field to autopopulate a field in a newer form, but I can't. The value there is a string and the new field -- also a lookup -- wants the ID.

Given that this particular form has been in use for three years, there are many records affected. I suspect the only real solution would be to create a new lookup and try to create a custom action that will populate the new lookup by matching the field value to the appropriate record ID, but I'm intimidated by that because my Deluge skills are pretty weak. Is there any other way to go on this?