I have been using Futaba radio, receiver and servo, but I am considering a Spektrum radio system ... is the plug (J?) on my Futaba servos going to fit into a Spectrum rx (3100 or 3500) as is, or is it going to need to be changed?

Just need to trim the little side tab off of the Futaba plug so it will fit. Then make sure you have the plug in the right way. For a 3100 the black wire to the outside of the receiver, believe all Spektrums are the same, and they are marked with symbols.