FlyerTalk Evangelist
Join Date: Aug 2011
Location: Barcelona, London, on a plane
Programs: BA Silver, TK E+, AA PP, Hyatt Globalist, Marriott LT Plat, Hilton Diamond
Posts: 13,029
You can upgrade whenever you want if there is J space available. Works pretty much the same as the Elite Plus upgrade vouchers.
If there's no J inventory, you need to wait until check-in, when you can access any available seat up front...