Qatar has a nasty habit of swapping planes at the last minute, on these Qatar Pacific flights perhaps there is slightly less chance as the planes seem fixed to routes (HKG, MLE?), but they don't have many of them so if one goes tech or is delayed a lot on the inbound you'd be stuck.

If they did the equipment swap:

• In advance, don't think there is much you can do (I had F booked on an award ticket and then they pulled it from the entire schedule for months, though this was back during COVID as they seemed to have decided to redirect the planes, I had the Qatar Pacific plane on IST-DOH about the same time as my original ticket). I was just automatically downgraded to J by the system and my ticket reissued, no first class lounge access given or any other "sorry".
• If on the day I guess you can decide then whether you want to wait a day for the next flight 24 hours later, or get on the original timing in J. Hopefully that would happen by the time you were in the first class lounge (not really worth that much more IMHO, but something for nothing), but it is impossible to really guess when they may do it. Per many other on this forum complaining and my own experiences, equipment swaps can happen only an hour or two before the flight and you would only realise when you go to board and get a new seat number (not sure how this would happen in the F lounge, since you board from there).

Since the program you are booking with doesn't charge a difference between J and F for points I guess you don't get a rebate, there is no EU compensation in this case and I wouldn't expect Qatar to do anything, so you have to hope the equipment sticks! Personally (and I do have a Qatar F on this route booked later this year but paying many more Avios for the seat) I would rather have QSuites consistently than the current sub-premium aircraft that we get on the HKG route at the moment.