Happens regularly, it's unfortunate but nothing you can do. Are return flights on days adjacent to the one you booked (+/- 1 day) also changes to a 77W? You haven't mentioned cities so we can't really tell, but if it's a one-time switch EK might allow you to change to a day earlier or later.

An airline reservation is only for A to B in a given class of travel, never for a specific aircraft type. When did you book, the further in advance you book the bigger the chance of an equipment change. Also any flights straddling October and I think March or April, as thats when summer/winter schedules change is more likely to have this happen.

With the US' recent blatant attempts at stifling competition from ME carriers EK has been hit hard and downgauged several routes.

Also who knows, you might get lucky and get a new J in the 777. Fully flat, shiny and new. About 7% of fleet has it and they often fly to US routes AFAIK.