[Django]-Customizing an Admin form in Django while also using autodiscover

None of the above. Just use admin.site.unregister(). Here’s how I recently added filtering Users on is_active in the admin (n.b. is_active filtering is now on the User model by default in Django core; still works here as an example), all DRY as can be:

from django.contrib import admin
from django.contrib.auth.admin import UserAdmin
from django.contrib.auth.models import User

class MyUserAdmin(UserAdmin):
    list_filter = UserAdmin.list_filter + ('is_active',)

admin.site.unregister(User)
admin.site.register(User, MyUserAdmin)