exchange-point-and-mark would swap the ends of the selection in caret mode or when selecting text in a
text box.  It would be bound to C-x C-x as in emacs.

I have found that some selection modification commands don't seem to allow an explicit specification of
which end is the point and which end is the mark, and rather merely allows a start and end position to be
specified.  Maybe there is a lower-level selection interface that doesn't have this limitation.