No, they are not backwards compatible. There are some differences in the Rabbit 6000-based MiniCore modules and you need to use an updated development board:
This PDF describes some physical changes between the RCM67xx and RCM57xx boards.
I don't know specifically what hardware updates are necessary for existing customer-designed baseboards to work with the RCM67xx. If you need that information, I recommend contacting Digi Technical Support.