Django - best way to manage existing, auto increment fields with existing data?

Question

This is still in dev, so I am relatively flexible in how to approach this.

So I'm importing data (via fixtures, the import works fine) from an existing database into a django application. The source data is not uniformed in how it manages ID & primary keys. Some tables use what seems to be an auto-increment (similar to what django would produce by default). Others use some sort of integers. The relationships in the data dump are established based on those fields. Seems I can keep on using auto-increments in all cases.

They are conveniently not named uniformly: id, pk, pk_sometablename, etc.

The fixtures I use to import look like this (I generated them using a script based on the datadump, so this can be changed if needs be):

{
    "model": "admin_account.client",
    "pk": "168",
    "fields":
    {
        "pk_client": "168",

My django model:

class Client(models.Model):
    pk_client = models.IntegerField(verbose_name='Pk_client', blank=True, null=True)

I need to be able to import the data in such a way that this field, the pk_client field is used as the primary key (it can still remain as an auto-increment). So I tried to change to this:

class Client(models.Model):
    pk_client = models.AutoField(primary_key=True, verbose_name="pk_client", default=-9999)

However if I try this migration with my currently populated dev DB, I get an error:

django.db.utils.OperationalError: foreign key mismatch - "purchase_orders_apent" referencing "admin_client"

I assume django complains because the apent table used to try to lookup client.id, and since I know tell django to use pk_client as the primary key, perhaps those tables referencing it are now unable to find their match). But there's a lot of tables involved.

What the easiest way to manage this?

Should I completely define those models with an empty DB (e.g. define the AutoFields for each model, assign the old db auto-increment value to that same field, and only THEN import the data)?

Or is there something I should change in my fixture definition/the way I import the data?