I have not had SRS, but I also stopped standing, basically as soon as I decided that transition was a thing. It was weird for about a week, but now the thought of standing or using a urinal really doesn't even occur to me. It's been about 4 months now I guess.
It is still a bit annoying, because it's hard to get the last little bit out of the pipes until I stand up, but I'm dealing with that.