Well if you are ticketed in J, and have seat 1A pre-assigned, then no-one is going to take that away (well, barring a resurrection of the Seat Shifters, but extermely unlikely).
Secondly, if you are you are waitlisted in Y under the same reservation, I guess you can keep checking MMB to see if it confirms. You would need your ticket re-issued, which will cost about £792 in service charges. Is it really worth it?
Whether you can continue to waitlist in Y after checking in in J is an interesting questions. Logically not, as OLCI implies a firm intention to travel, but may be the systems aren't that clever.