I would just do a few at a time, that way you only risk losing a few. I made the switch with plowing customers, and lost about 20%. But it worked out for the best. I found that it worked better if I did it in writing at renewal time...it gave them time to think and time to compare me to the competition. Most couldn't find a better deal, so they stuck around.
It just depends on how much you need the customer vs. how much they need you. Supply and demand!