Why not use your Avios to redeem directly into J?

"Upgrading using Avios" is not a good strategy if your requirement is to actually travel in J. It's just there so that, if you happen to be stuck with a Y ticket, you can "try your luck". It's a lottery, a gamble - there never is a guarantee that you can secure an upgrade - particularly when you have to commit to a Y ticket to even enter the lottery.

If there is no J availability for such an outright redemption, I think you can take it as 99% certain that there will be no upgrade space and any request for an Avios upgrade will be rejected.