This depends on one major factor: are the shirts symmetrical in cut from front to back? That is, is the sleeve opening, sleeve shape, and shape and size of the front and back panels identical in front and back?
If they are basic T-shirts, the answer will likely be “yes,” though the sleeve shape may be slightly different from front to back. I would try them on backwards and see how they fit before proceeding.
The front collar is always going to dip lower than the back collar, so you will have a slightly lower than usual collar in the back unless you add another material to make up the difference (this could be done in a decorative way, I suppose!).
The simplest way to alter these would be to cut the back collar so it matches the depth of the front collar (or the depth you want it to be), and then sew on a new strip of fabric to finish the edge. Look for a knit in the same color (or contrasting, if you want to be fancy) as the shirt, make a strip, fold it in half, and sew it on, much like how a commercial T-shirt collar is finished. A rib knit would work well for this, but it doesn’t have to be.
If you want to get fancy with the new collars, go for it! You can turn them into boat necks, or if you wanted to get rid of any sleeves/the sleeve shape was too different front to back to comfortably wear, tank tops, with the back (now front) armhole cut a bit lower and further forward.
I hope this helps :] Good luck!