FYI - I was unable to upgrade my ticket
In the end, I had to purchase a comlpetely new J class ticket (ouch!)
I will see if VS will refund my 'non refundable' Y class ticket out of good will considering I spent just a FEW extra pounds on the new ticket.
Will report my progress.
I must say, though, I am surprised that I just couldn't pay the upgrade fee to upgrade my Y class ticket.