From what i've seen if you ask the check-in agent that you want to upgrade, they put the request in the system but wont give you the upgrade until the flight is not closed for check in, incase of any J class ticket holders showing up without a booking....

However unless the J class is very open, they might release a few seats at check in...best suggestion for the OP is to ask at checkin and then speak to the lounge agent when they arrive in the louge (provided they have lounge access). If not, chances are you wont know till the boarding gate.....