The reason wireless providers can't do it is because they don't have bench certified technicians. In the U.S., I don't believe any of the major providers have technicians at all any more, let alone people certifed by any manufacturer to open a phone. And, if a phone is disassebled by anyone not so certified, the warranty is immediately void.
If you don't care about voiding your warranty or right to an out-of-warranty replacement from Apple, you can always try to do it yourself. I do NOT recommend you do that as the end result can be very sad. And that's why wireless providers won't do it, either.