Trying to replace a char field with a forgein key feild

So when we started the database, it was good enough to have a char field with choices. But as time progressed I need to convert that field to a foreign key field. The follow I'm trying to take is such:

1. rename <fieldname> to tmp_<fieldname>
2. create a new field with <fieldname>
3. have a RunPython fill the data for <fieldname>
4. delete field tmp_<fieldname>

I can get steps 1 through 3 to work with out issue. But once I remove the old field from the code, the migration breaks because the field no longer exists (which makes sense). I'm curious if there is a way to work around this, because my googling is coming up empty.

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