Well... I'm not on v4 at the moment and can't give you specific information on the rewrite function it has (because it does have SOME function, I just forget what it is) but my tip would be to do it the way I do when I need to do something like that - change the color or symbol of those two positions, so you know who they are and can easily identify them from page to page, grab your lasso tool (or just Control+click each of them) and just go through each page in order and it's a 4-part process per page:
Copy + Paste + Flip + Accept.
Once you get going on it, it should go fairly quickly and save you time.
For me, it's the Ctrl+click I would do (rather than lasso), since my pinky would just stay on that key (I hope it's Ctrl for you... might be Command if on Mac and using the Mac shortcuts...) and then once selected, holding the Ctrl key I hit 'C' then 'V' then in the window that opens I click "Flip" and then "Accept".
Simple... not the built-in rewrite method, but it's the best for me, especially if it's only a couple spots.
If it were 3, 4, 5+ I would probably go through each page individually and manually rewrite those. But if the positions are flipped exactly, than the Copy/Paste/Flip method would work out just fine. Had to do this with the bass drums a couple tunes ago as I inadvertently flipped them on a set and had to go through and fix that. All I did was select em, copy/paste/flip/accept - done!