I have lost the ability to do so in both my dev and prod bot. The Knowledge URL is completely greyed out. After specifically choosing "edit" on the source.
I know I was able to, at some point.
I can edit public website URL's still.
This is terribly annoying because, if I have to delete the source and recreate it, then I need to go through every topic that was using it and re-add the source and its time consuming.