Emirates seem to have changed the redemption classes recently. I'm pretty sure that you used to be able to upgrade as long as there was room in the cabin you were upgrading to.

However, it now seems that they have a seperate bucket for upgrades.

To book the upgrade you phone the Skywards centre. You have to pay an extra £20 tax for the J class upgrade.

I'd phone Skywards first, and check that there is upgrade availability before you book the Y class. Maybe stay on the line while you book online, and then get them to upgrade straight away.

Cheers,
Rick