A few years ago I was invited to a game by someone I'd met travelling back from London. He had 2 Main Stand season tickets, for him and whoever he invited to a particular game. Both were OAP tickets - but he simply went to the box office before, and upgraded it. (In the event there was no-one checking as you scan in, but concessions beep and the light is a different colour - much like Underground tickets). So sure you'll be fine.
I think a few people do this - buy 2 tickets and go with random mates each game. Buying the concession ticket makes sense as you can upgrade, whereas if you had a regular ST but took an arl fella, you can't get a partial refund.