Django - remove primary_key=True from an existing model? - mysql

Question

I have a table that I've already migrated into the MySQL database and one of the fields has primary_key = True. I realized this needs to be different and I need to remove the primary_key = True. I'd like Django to create its own auto incrementing primary key fields (like it does with all models if you don't specify a primary_key).
If I simply remove the primary_key = True, Django complains because it can't auto-assign an auto-increment value to its new id field (which is now the primary key field).
What's the best way to change this one field to not have primary_key = True? This particular model doesn't have any records yet in the database, so I think I'm in a better position than if I had records in there. I'm not sure if I should just drop the table and migrate as if it's a brand new table or if I need to take some other approach?
Edit
What I actually tried:
python manage makemigrations accounting
The model in question is called Invoice and I'm wanting to change the field inv_num to not be the primary key
Django asks:
You are trying to add a non-nullable field 'id' to invoice without a default; we can't do that (the database needs something to populate existing rows).
Please select a fix:
1) Provide a one-off default now (will be set on all existing rows)
2) Quit, and let me add a default in models.py
Select an option:
So I select option 1, and type 1.
Django creates the migration file, so I do:
python manage migrate accounting
And Django complains with:
django.db.utils.OperationalError: (1067, "Invalid default value for 'id'")
My research indicates that this is because you can't auto-assign a value to a primary key field.
Edit 2
The Model in question:
class Invoice(models.Model):
inv_num = models.CharField(max_length = 144, primary_key = True, verbose_name = 'Invoice Number')
brokerage = models.ForeignKey(Brokerage, blank = True, null = True, verbose_name = 'Brokerage')
lot = models.ForeignKey(Lot, on_delete = models.SET_NULL, blank = True, null = True, verbose_name = "Lot")
vendor = models.ForeignKey(Vendor, on_delete = models.SET_NULL, blank = True, null = True, verbose_name = 'Bill To (Vendor)')
contact = models.ForeignKey(Contact, on_delete = models.SET_NULL, blank = True, null = True, verbose_name = 'Bill To (Contact)')
due_date = models.DateField(blank = True, null = True, verbose_name = 'Date')
fund = models.ForeignKey(Fund, on_delete = models.SET_NULL, blank = True, null = True)
org_amt = models.DecimalField(max_digits = 12, decimal_places = 2, default = 0, verbose_name = 'Original Amount of Invoice')
amtos = models.DecimalField(max_digits = 12, decimal_places = 2, default = 0, verbose_name = 'Amount OS')
is_fine = models.BooleanField(default = False)
is_reversed = models.BooleanField(default = False, verbose_name = 'Has Been Reversed')
is_paid = models.BooleanField(default = False, verbose_name = 'Is Paid')
is_locked = models.BooleanField(default = False, verbose_name = 'Is Locked')
is_archived = models.BooleanField(default = False, verbose_name = 'Is Archvied')
org_je = models.CharField(max_length = 144, blank = True, null =True, verbose_name = 'Original JE')
There are a number of other models in this Django App that have Invoice as a foreign key. But those too don't have any data in them in the database.
I wonder if I should just drop the whole app from the database, and re-migrate it in? I do have a bunch of other apps that have data I cannot simply drop though.