PremEx2000,
Change of gauge is purely done for marketing reasons, to be able to market more direct flights.

Ozstamps,
Actually UA1/2 used to have a change of gauge in DEL. For a while, the LHR-DEL segment was a 767-300, and the DEL-HKG segment was a 747-400. But the reborn RTW service is supposed to be a 747-400 all the way (at least at first).