You can't specify the booking class but you can phone up and book.

If there is G/O/Q availability then, by definition, there is also N availability. This would mean you'd only pay half the avios but would earn avios on the original booking (1300 or so for you as gold) and would keep your reservation.

As for whether you should cancel and rebook - UUA is clearly better if it's an option - phone and check. But you're still getting over 1p/avios so while it's probably not the biggest bang per buck (that amount would upgrade an LAX-LHR from premium economy to Club for one person) it's an OK cash value.