It depends if you buy a controller and and receiver and its frequency is FM and the servos are AM then it wont work make sure when you switch controller and receiver that the servos are the same frequency. i had this problem my LHS sold me a FM servo when my receiver was AM
Standard servos don't care whether the radio is AM or FM... as long as they'll plug into the new reciever (and the Rx modulation and frequency match the Tx) they should be fine. It's only with some of the higher-end servos that you have to start getting specific kinds.