A moment of thought on this and without a diagram to aid me I think the following should work....
you only need to swap the collector and emitter around, base does not change so does not need any switch connections though a daughter board with flying leads might make things a little bit easier to manage, so....
DPDT on-on switch
centre lug on each side (Pole) of the switch are the connections to the board, one side (Lug 2) to where the collector would normally go , the other (Lug 4) to where the emitter would normally go
link between lugs 1 and 4
link between lugs 3 and 6
these will look like a cross
connect the collector to lug 1
connect the emitter to lug 6
this is assuming the lugs are numbered like this
In one position the centre lug is connected to the pin above it and directly to the transistor, in the other position the centre lug is connected to the opposite side of the transitor via the link vi the bottom pin. In the first position the links do not make a connection to the transistor. I would think that switching will cause noise or popping so might be worth muting or turning down the output volume or amp beforehand.