Clive, I have just done it the other way around, ie Removed the GSA racks and replaced them with the vario's from my GS.
You will need to fit the GSA footpeg mounting brackets, it's a very straight forward swop over, the bikes chassis are the same, you are just swopping bolt on bits. The top rack mounting holes are already threaded to take the top rack screws.
If you pop the following title into the Ebay search bar you will find a brand new set of brackets exactly what you want, however they are a bit pricey as they have the actual pegs included, you don't need these as you already have the ones from your own bike. Try do a deal perhaps....
search ebay for.. BMW R1200GS K50 K51 Rear pillion foot peg hangers NEW 2013 - 2016