Here a good write up on the interplay: [https://support.apple.com/en-us/HT204154|About Thunderbolt ports and displays]. Sadly, a Thunderbolt display will not play with a DisplayPort based system.
As to altering the display, I think its wiser to sell it and get a DisplayPort display. The other option is to see if a Thunderbolt card installed within your MacPro will work. It was on my list of projects to do some day never got a chance to try it.