I was able to upgrade J to F recently on a Business Flex+ ticket at the airport and it worked out great, and the website is quite clear about that. But, saver tickets have different wording:
You can upgrade a Saver fare using miles as long as there are seats available. In Economy Class, you can upgrade at check‑in or on board. In Business Class, you can upgrade at any time up to six hours before departure.
Does this mean that a saver award in J cannot be upgraded at the airport? I find it odd that they say you can do it up to 6 hours before but I am pretty sure auto upgrades expire at 48 hours. This also contradicts itself by saying you can upgrade at any time as long as seats are available, but then puts the 6 hour limit on the business ticket.