I've had this before more than once.
Typicaly where an ssfp was fitted to the nte with a data extention to get the dsl to a router in one office.
I fitted a 2nd data extention kit needed to get a router working in a different room, leaving the old extention kit in situ. If the customer wanted to switch back to the other original extention then they can unplug one and plug in the other. leaving which ever unused extention completely disconnected from the whet wiring.
As you have the DSl extention hard wired I would disconnect this from the IDC leaving the wires free in the socket in case you need to reuse them, then connect the new DSL extention
If you think it's going to be a case of switching between one and the other extention on a not infrequent basis then maby converting this so you plug in the extentions with an RJ11 into a typical ssfp would be better.
Although this would mean fitting a RJ11 plug on the end of the cable, and I would imagine you won't have enough slack on your existing cable to do this and still fit the plug in the front plate. so maby either a very short extention into a new jack with an rj11 socket. This giving you enough slack to use the existing cable with an rj11 plug.
Otherwise Black sheep's idea of a switch. I used to have an 8 pole double throw switch fitted inside my test nte
(so I could put an rf3 in and out of the circuit for testing at the flick of a switch. And if the rf3 was off then its completely disconnected from the circuit. Oh and Blacksheep...I had one in my DACS remote, with input and output jacks for the test leads. and butt phone. to go between B1 & B2 I just flicked the little switch on the end. Very handy, especialy when up a pole or in a confined area)
This might work fitted in a junction box
http://www.maplin.co.uk/4pdt-toggle-switch-2343 Its a smaller version of what I used an 8pdt.
The whle problem is that anything tee'd off the wiring is an potential for interference and if you imagine the wires like a small brook. Any ripples (the dsl signal) would flow along the waters surface. If there is a small bit of water forking off to a dead end, then the ripples would reflect back from the end and further reflect back on to the original ripples in the main bit of water. You'll see the original ripples with other ripples comming back interfering. anything tee-d off would cause this with any signal in the wiring. Just as mentioned with the Bridged Tap.
This is a common problem caused by extention wiring in a typical home telephone network. So it's not just interference which internal wiring throws up for DSL
Whats even worse I've found is if the internal wiring comes back on itself, forming a ring. Just about guarenteed to cause a loss in performance
So if you have the means to keep your DSL signal running along one pair of wires as simply as possible with as little added to it as possible, the better.
So I would avoid leaving the old dsl extention connected, especialy as its unlikely that you'll need to switch from one to the other often