[Answer]-How to remove unique constraint using South?

In order to solve this I did the following:

1. Remove the unique constraint, run schemamigration and migrate it.
2. Remove the null=True from tax_code, run schemamigration and migrate it again.

Step 2) has shown a Truncated data warning, but it ran fine. Re-migrating it doesn’t show any errors, so everything seems fine!

Hope this helps somebody else too!