Keep in mind that you will also have to relocate the gas ports.
This is not true if you're cutting a 6920, which uses a carbine-length gas system, to 10.5" which would still use a carbine-length gas system. In order for it run reliably though, you may need to have the gas port diameter opened up.
ADCO charges $65 to have the barrel cut, threaded, and recrowned to your specs if the barrel is uninstalled from the upper. I believe they charge a bit extra if you send in a complete upper to have work done that requires disassembly. From what I've read over on AR15.com, they get nothing but good reviews as well.
A link to their forum on AR15.com:
and a link to their website: