If you want the R7 to just plug in and play if you pull the R6 out, you will need to pre-program the R7 independently. You would also have to slide the ext. cables X18- the end all one port lower on the system, and the CO lines up toward the proccessor. The R7 has 5 CO line ports, and 9 ext ports. So if you do not slide the cables around, X10-17 will be the same on the R7, but X18-the end would all be off one ext number higher. For the CO lines, if you do not slide them down to the open CO ports would be the same for CO lines 1-3, 4-5 would have no dial tone, and the next CO line would be line 6- however many more lines you have.
I would recomend getting two R7's, or two R6's, if you are looking for one to be a back up processor. Unless your cables will reach, and you can figure out how to slide the cables around and pre-program the R7 independent of using a back up, in order to restore it to the R7.
You do not always get what you pay for, but you never get what you do not pay for.