Using Avios to upgrade is really only useful for someone who is forced to travel in Y. If you've no say in the matter, and are provided with a Y ticket, then there is no harm in checking to see if your Avios can get you an upgrade (and hoping that the stars align).

No matter the airline - except for BA who have a process where you can see at the time of booking the Y ticket if the upgrade is available and will go through - I would always advise to completely ignore the "upgrade using miles" option unless you are in the unlucky starting situation of having no say in the matter and having a Y ticket imposed on you.

However, if you are the one in charge of all decisions regarding this ticket, and if you want to travel in J - and particularly when J seats are available outright and you don't need to resort to buying Avios - it's always better to redeem directly into J with your Avios rather than doing the two-step process of locking yourself willingly into a Y ticket from which you later may never be able to escape.