When I first started offering my hosting service, I packed with with features and lots of bandwith and space. Not unlimited, but a lot. Then I noticed the actual usage was between 1/10 to 1/20 of what I was actually offering. That made no sense to me. So I sent a survey to my members asking them if they would find it acceptable to have plans that are closer to what actual members use instead of paying for something they don't use. I reduced a few of my plans and added new benefits that they did not have before.
The important thing is make it sound like its a business decision to avoid having to increase prices and/or reduce customer service. Don't be rude and don't use a YOu have too approach.
Also, you can offer them at the end of their commitment a new plan with a special discount or gift and tell them that the old plans have expired.
Also make sure to remove the unlmited mention off your site. Offer your exisitng customers more space, more emails, something to make them feel special.
JMO... But it worked for me.