Dump existing data:

python3 manage.py dumpdata > datadump.json
Change settings.py to Postgres backend.

Make sure you can connect on PostgreSQL. Then:

python3 manage.py migrate --run-syncdb
Run this on Django shell to exclude contentype data

python3 manage.py shell
>>> from django.contrib.contenttypes.models import ContentType
>>> ContentType.objects.all().delete()
>>> quit()
Finally:

python3 manage.py loaddata datadump.json