The answer is yes. Basically the timer outputs a servo position signal and the position is in essence a ratio demand of the supplying battery voltage at the ESC. Without changing also the motor the voltage output will need to be reduced from the new battery to the old battery voltage value, ie turn down the timer output. Think of it like this, the timer puts out a signal that goes from 0-100% of the Vin or for a 14.8V 4s battery 75% would be 11.1V and for a 18.5V 5s battery 75% would be 13.9V. So, without changing the motor to maintain the same speed the timer output has to be reset to 11.1V/18.5V = 59.5 %, "turned down to 59.5%". In other words the pot on the FM0c would have to be turned down from roughly 3/4 to just under 5/8 turn thereabouts.