I'm sure this has been answered from Jon somewhere, so while I don't have the authority I think I can answer.
Dopus 9 users were able to get a discount to version 10 for the three years that version 10 was the latest, so until version 11 came out.
I suspect that regardless of what version is current (11, 12...) or time since release. Registered users with a license to previous version (current -1) will always be able to update the current version at a discounted rate.