I received a reply to my complaint message. The standard Golds-get-50%-discount thing. I specifically asked about upgrades of Golds in the case of regular economy being full, and Up front seats being available.
In cases that all standard seats have been occupied by other passengers, when you complete your web check-in, you shall be offered at no cost one of the available seats randomly. It should be clarified though that, in case you wish to change the automatically given seat to any other chargeable seat, you may pay the relevant cost.
So no preference to Golds, and no way to move to another seat if you get stuck on a middle seat.