There is nothing to prevent you from rotating the bar end shifters, though it is a bit of a pain for the rear shifter. For the front shifter just loosen the cable off the derailleur, move the handle out of the way, loosen the expansion bolt using an allen wrench, rotate, and put it all back together.

For the rear derailleur, you have to do the above and make sure you get the little spacer back with the right orientation. That is a pain, but we can step you through it.

If you rotate the shifters, the cable will now come out of the top of the shifters. That would seem odd to me, but it would be no real problem once you tape over it and get used to it.

I would just leave well enough alone, but that is just me.