Notify them in March for new prices in April, for those staying the same let them know you have done a yearly price review but will remain the same for another 12 months.
This makes customers aware that prices are looked at yearly, we normally increase about a third very year, might lose 2 per hundred
It's your call, if a person is underpriced and you hate doing it OR you make more by getting better priced work then if they leave so be it.
If you want to keep working cheap, then, just keep cleaning them.
Darran