Our upgrade docs look like they are missing a step. Could you try upgrading to 21.5.0 before going to 21.6.3?
21.6.3 no longer has the south to django migration code. Going to 21.5.0 first will let you get off of south migrations and onto django migrations. 21.6.3 is important to stop at as that release will get you past the last migration that was squashed in 21.7.0