Colin,
If you are dealing with same set of users assigned same roles for same time validity, then you can simply first remove all the roles in SU10 (remove till 12/31/9999) and then add them back to the new time validity dates.
If you have n users assigned m number of roles (different users different set of roles) then there is no SAP delivered way, best you can do is to develop a custom program to do it.
Regards,
Shivraj Singh