I suspect RT probably permanently changes bowel habits in most cases, but not necessarily for the worse. I go twice as often as I did, but that's not a problem, and I can hold on with no problem if it's not convenient to go.
After I'd got over the immediate effects of the RT, I found it was much easier to go for the next year or so than it had been before treatment, probably because there was more rectal mucus so anything slid out easily, but not enough for the mucus farts (as my oncologist calls them) any more.
Apart from tending to go more often and occasional blood, that's all now back to how it was before treatment.