What's important to remember is the nature of slots at LHR; they are virtually priceless and Cathay have 5 daily slot pairs. They're, unfortunately, not as competitive with 6th freedom traffic (e.g. LHR-SYD *via HKG*) due to worldwide political situations (to cover all bases). Are CX going to sell a slot pair or fly their smallest widebody (for the pedantic ones: they aren't putting an A330 on LHR so just rule that out)

Just like with W23, these are fixed for S24 in terms of planning but 'never say never' and there's a chance these aircraft can be swapped in/out (e.g. A359 <-> A35K) if so be necessary.

As for Aria, it is not advised to book just for the product at this point. As oldchinahand mentions, it's purely speculation and you'll only be disappointed if it isn't the Aria suite. It is fully plausible that they will swap out one of the A359s for the Aria Suite 77W's if they want to increase the overall premium cabin capacity on the route (assuming the new planes have at least 40J/32W like the 3-class 777s, since the A359s are 38J/28W)