I want to know if its feasible to change the membrane thats mounted to the motherboard as one of my keys no longer has the noticeable "click" or bubble on the membrane that is there for the other keys.
I have another broken 9000 for parts but wanted to know if anyone had ever swapped over the membranes (not the keyboard itself as thats not what makes the click when you push a key).

Anyone got any ideas ?

It can be done. There is a thread from a few days ago from a member discussing how he fixed the multiple repeated character issue on his 9780 by replacing the membrane. Have you looked for tear down videos for your specific model?

maybe this will help:

Curve 8530 keyboard/trackball issues