While it is technically possible to re-chamber your 223 Remington AR15 barrel to 22 Nosler, it is far more difficult than a typical barrel chambering job and not within the ability of most gunsmiths.  The 22 Nosler was designed with the cartridge shoulder being somewhat shorter than the 223 Remington in order to prevent a 223 Rem cartridge from chambering in the 22 Nosler chamber.  For this reason, a 223 Rem barrel would need to be set back before re-chambering to ensure it properly cleans up.  Due to the clocking of the AR15 barrel extension and gas port, this is a very difficult task.  There are many drop-in barrels and upper assemblies available in 22 Nosler that take advantage of the AR15 platform and it’s modularity, and we recommend people follow this path.