There is a huge difference in the space available in J and Y, and while the CX service in Y is excellent, the seat pitch is only average. If you are tall/large or have any difficulty with small seat ptches then I really would not recommend that much flying in Y in such a short time. (We are talking well over 20 hours in each direction.)

The problem is that if you downgrade to a Y award (and only spend the Y award miles) then CX will see it as just that. CX has no mechanism for upgrading AAdvantage awards "on the fly". (If you were doing this with AsiaMiles then you would have more possibilities.)

As I understand it, AA members cannot waitlist for awards on Cathay (again if using AsiaMiles with CX status you could) so I don't see any way you could get "standby" status.

Sorry to be the bearer of bad news...
Have you considered going in First? CX First is the best there is.