Just out of interest, if you're going to fit a back brake and therefore not going to be doing skidz any more, why do you need to change from fixed to SS for the sake of your knees?
I guess it depends on your route. I don't enjoy fixed downhill (and obviously uphill it makes no difference), but once you've got a back brake I find it actually more knee-friendly than my other non-fixed bikes.
Just out of interest, if you're going to fit a back brake and therefore not going to be doing skidz any more, why do you need to change from fixed to SS for the sake of your knees?