I always wonder how to handle existing customers with such raise. Have you just sent a mailing and started charging double price from their CC the following month?
I ask first. If they say "no", I just let them stick with the old price. I may then ask again in 6 months and see if I get a different answer. I also sometime just cancel the billing and let them continue to use the service with the understanding that there will be no support. You gotta experiment.
You should keep them on a grandfathered plan for at least a period of time and/or nag them to upgrade (optionally with a discount for their trouble).