Yeah I've just had this pointed out to me by support.
Seems you need at least 10 Service Credits to get them to upgrade an array.
Possible discounts for addition arrays or additional credits for out of hours.
I think I missed the memo on this.
Luckily only got one array not at 3.3.1MU5 and have been doing the basic patch installs on some other ones for a while so am okay with that. Just working on a plan to get the last one updated from 3.2.2EMU4, appears to be not too many steps from the vague details I can find.
Have mentioned to our Proactive support TAM that I'm not too pleased, seeing as it was one of the few support things we used. Maybe they'll refund some support costs as service credits.