After market upgrade of the new MacPro (late 2013) CPU?
While there is no teardown of the 2013 MacPro yet, digging into Intel's Ark DB might be useful. 

For example the i7 processor used in the MacBook Pro Retina late 2013 is a 4750HQ registered as using the BGA "socket" (Ball grid array) which is synonym of soldered to the mainboard. 

For the MacPro Apple tells us there is either a 3.7Ghz w/ 4cores or a 3.5GHz w/ 6cores or a 3.0GHz w/ 8-cores or a 2.7GHz w/ 12-cores. All the processors that have these specs in ark are listed to only using the FCLGA2011 socket (see here the Xeon E5 v2 spec sheet). 

So to me it's pretty sure that yes the cpu is not soldered to the mainboard and is therefore upgradable.