I've been searching a lot about cpu upgrade and I have some good and bad news. Good news first is that yes you can change both single and dual core processor 2012 Mac Pros. Bad news is that they should be Xeon, and as mentioned before, there is issues with i7 since the architecture was built based on Xeon. I prefer Xeon than i7. Now the Mac Pro uses an old and retired socket which intel does not make cpus with those sockets anymore. You can barely find new ones on amazon and most of them have reached "end of life".
Go to intel's website and in Xeon processors look into the 5000 series, those are the ones with LGA1366 socket. However the nMP comes with the newest version of socket, the LGA2011 and you can change the cpu there as well...with a bit more hassle. I really wish apple had kept the current design and just upgraded it entirely!
But hope that answers your question!