Not sure on the reasons behind the change, as it may be because of the capacity adjustment that is making JJ retain their 777 for longer than they expected and to dry lease some of the new A350's to QR. It could be a mix of that and seasonal (but it's definitely not an operational change, given than the flight is more than a month away yet)

IME, JJ triple sevens are not as nice of a product as the A350s... the interiors are quite outdated, the IFE is rather old and you don't get the perks you'd get on an A350 (which you wouldn't get on any 777, retrofit or not) such as lower cabin altitude, bigger windows, less dryness in the air after a long flight, lower noise levels, etc.
I find the 777 is a comfortable plane to fly in though, but you'll feel you're flying 15 years ago.