South migration when custom field definitions change

[graeme]

I have an app that uses some custom model fields. These are subclasses of standard fields.

I have tried to include as many defaults as possible in custom field to stick to DRY.

I have written South introspection rules for these that work fine when I change a model

What happens when I change the custom field itself?

For example, if I have a subclass of CharField with a default max_length, and I change the

default max_length in the custom field? What if I decide I want no

max_length and decide to subclass TextField instead?

South cannot see the change and running schemamigration --auto gives "nothing seems to have changed"

Is there some way to create a migration easily, or would it require creating a migration manually?