I don't think you can now as it's seen as essentially transferring the ticket to someone else, which can not be done out of fairness to the thousands on the ST waiting list.
You'll have to call supporter services, but I wouldn't be doing so with any real optimism.
Somewhat foolishly probably, I've just taken a look at the various Ts&Cs that you've linked to.
Whilst you're quite correct in what you have quoted above, it is from the "HOME MATCH TICKET CONDITIONS OF ISSUE" which I interpret as being single match tickets.
Under "Season Ticket Terms &...
I don't want to keep posting about this as every post I make increases the chances of me getting a stroppy email.
Transferring football tickets to someone, even if no money changes hands is illegal. The club has repeatedly told us about this.
However, many friends and families share tickets on...
They can't enforce it without photographic ID at the point of entry.
That aside, you've sort of answered your own question. Ticket transfers are allowed, legally, if the club authorise them. So, when ticket upgrades are processed, the club is providing agreement for that to happen. In the same...
Covered here: http://www.northstandchat.com/showthread.php?353691-Reminder-to-Supporters
It's just a continuation of the club's messaging over the last few seasons.
The simple fact is if you give your season ticket to a friend or family member to use then there is nothing the club can do about...