I don't think thats right, it appears to me that AMD are producing 2 Semprons a socket 754 one and a socket A. Watch which one you buy. The XP processors are all socket A but are very differcult to get now. Wait till someone confirms what I have said is correct but looking here for example click here you will se there are 2 listings, one for socket A and one for socket 754 and they are different prices.
Never seen a 939 one but theres your answer simonsup, if you want to replace a socket A XP processor with a Sempron you can BUT you must make sure you buy a socket A one and not a 754 one. If you want to upgrade your processor you'd better go to your motherboard manufacturers website and see if it'll accept it before you buy. And check the BIOS number it needs as you may need to flash the BIOS.