As previously posted the simple answer is yes. However that is assuming you are referring to a "complete" 7.62X39 upper. In addition to the barrel, the bolt for the 7.62 is different. Also, there is a magazine issue you need to be aware of prior to spending the money on an upper. Using 5.56 AR magazines works marginally at best. There are 7.62 ar mags out there most of which only work slightly better than the 5.56 magazines. Having owned, built and shot several 7.62x39 ars, the only magazines I have found which will consistently function properly, especially when loaded to capacity, are the USA "frakenmags." These magazines essentially are the upper portion of an ar mag welded to the bottom half of an ak mag. They are all "preban" and are expensive. Expect to pay $50.00 or more. Perhaps since the ban has expired USA will begin manufacturing these again. There is also a common problem with "hard primers" on surplus and wolf ammo. Low mass ar hammers often have insufficient energy to set off the primers. In addition, if you check the firing pin protrusion on an ak and compare it to the protrusion on a mil spec ar you will find there is significantly greater protrusion on the ak. I have had to remove up to .030 off the tail of the bolt to get consistent ignition of wolf and mil surplus ammo. Having said all this I still enjoy my ars in 7.62x39. But to confuse the issue even more the best solution I have found is the AR47.
DanS