The issue here is most probably the underlying economy fare being non flexible although I was pretty sure the upgrade costs were refundable if any change were to be made...
Have you inquired if you would take advantage of the LX goodwill policy for the voucher if the ticket + the upgrade cost would be included in this voucher?
I would suggest to wait until one of the flights is cancelled in which case it is LXs responsability of rebooking you (IN J!) on a date of your preference. (most probably tied to P availability).
As I currently see the situation I wouldn't want to decide this far out, especially since the circumstances change all the time. And considering that the current outcome (only addition of the price of the Y ticket and not the upgrade), will still be possible in a few months all you can do by waiting is improve your situation.