Yeah, there is a tab showing details of the flight and upgrade availability during the booking process. It is a bit laborious as you would need to check all dates one by one.

Another way would be to make a dummy oneway booking HEL-SIN with Adios in J. That way, you can see availability for the whole week. If it's there, then most likely that same flight in Y is upgradable to J (but check the tab in this case, too).