Are rear mech cages interchangeable?
 

I have a shimano 5600 rear mech GS(mid cage). Could I get the same model, a short cage (SS) one on eBay and just swap the cages and use it normally ?

http://si.shimano.com/pdfs/ev/EV-RD-5600-2452A.pdf
Yes, but not much point to it.

actually basically I’m thinking of getting it because I want to keep my 10 speed drive train as long as I can.

I fail to see any connection how shortening the cage of your RDER will do that.
You can always use a 9 speed "mountain" RDER and have the capacity to use larger cogs when you're aged.

I'm interpreting the OP's question as wanting to buy a spare derailleur for future use but the one he's looking at has a short cage and his current derailleur is a med. cage so can he swap the current cage to the spare when needed.

correct, should have made myself a little clearer! Still pondering if I should blow the money or just leave it since it still shifts very well. Just afraid of it getting damaged some day!

Spare parts insurance is always a gamble but if the price is right why not and you can always sell it for probably the same price later on if it doesn't get used. Seems to me older 105-Dura Ace 10sp parts are gaining value as most of the newer 11+ speed parts aren't compatible with 10sp.

Shimano's current Claris and Sora are still based on the old 6-7-8-9-10 spd geometry so parts availability is going to be around for many, many years. What's more, Microshift and other off-brand derailleurs also make parts based on this standard.

When 6700 Ultegra came out I bought a set of shifters and RD for my 6600 when places were clearing out 6600. They're still sitting on the shelf waiting for the day I need them or build something else. Also bought a 5600 RD that I ended up using on another build.

I have two rear mechs sitting on frames, one of which I will be disposing of and am considering whether to keep the mech. One is a Falcon on a 3 speed and the other some Chinese one on a 5 speed. The latter has an indexed lever and the former a classic friction lever.

Thus this kind of question is very interesting, but in the end I will try them out and see how they work out :)