This would seem to me to not be very probable that the servos could have been damaged by leaving the switch on, batteries yes, servos no. There has to be some other cause as such happening is very unlikely. Have you checked out the servos on another system or even on the old one now that you have the new servos working? Just plug them in place of the working ones, one at a time and they will probably work okay, just corroded contacts that got somewhat cleaned when you unplugged and replugged them. This assumes they were not in a fire or submerged in water or some such similiar happening.