I am in exactly the same transition situation that you are. I hooked up a TPDT-center off (triple pole, double throw) switch. If you have only one DC transformer, you only need a DPDT switch. Feed both lines of both power supplies to the DPDT switch input terminals. Connect the output terminals on the switch to your layout. Use the switch to select which kind of power you want delivered to the track. When I turn my whole layout on, both power supplies come on, but only one can send power to the track at any time.
To answer your question, with this system, there is never any DC feedback to the RailPro power supply, because BOTH wires exiting the RailPro power supply are turned off when the DC power supply is turned on. If I plan to run DC only for an extended time, I unplug the RailPro power supply to save wear and tear on it, but this is not necessary to protect it. If I plan to run only RailPro for an extended time, I simply turn off the DC transformers (using their on-off switches). But again, this is not necessary to protect the power supplies.
My DC layout has two cabs (transformers). I hooked up the RailPro to be toggled with Cab A. I ran the + wire from DC Cab B to the third pole on the triple pole switch -- wired so that Cab A and Cab B are both turned on and off at a the same time. This assures that both DC cabs are off when RailPro is on.
If you have DC block control switches (as I do) set all of them to Cab A when you want to use RailPro. If you have a DC directional switch, ignore it when using RailPro. Use the RailPro Forward/Reverse buttons on the hand-held controller.
The other potential problem is accidentally leaving DC locos on the track when running RailPro (or vice-versa). You need to be careful not to do that. If you leave DC locos on a Railpro track, they will immediately take off at top speed as soon as the power is turned on. No electronic damage, but likely crash damage. If you leave RailPro locos on DC track, they will just sit there and do nothing. But this is NOT recommended by the manufacturer.
P.S. I reread your post. It sounds like you are planning to put your switch between the wall plug and the power supplies. Don't do it that way. Put your switch between the OUTPUT terminals of your power supplies and your track. That will completely isolate the unused power supply from the used one.
