Here you go:
"We allow up to six upgrades because we fully appreciate that our younger fans may struggle to commit to every match, particularly midweek evening games that fall on a school night. At the seventh time of asking for an upgrade we will allow the season ticket to be fully upgraded...
The club published a policy on this. If you have used all your match-by-match upgrades (6, I think) then you could upgrade the season ticket for the remainder of the season.
So you'd have to pay the difference between the u18 season ticket and an adult season ticket pro-rata-ed for the number...
You can upgrade the ticket for the rest of the season. Means you're paying a bit over the odds when your son comes but ensures it can be used when he can't. If your "guests" pay a little extra it should cover your outlay, as they'll be getting a very cheap ticket relative to gate prices.