Had an email from Amazon.
Ordered 28th May, upgrade email arrived first thing this morning.
Upgraded my pre-order to the FIFA Mega Pack (PS4 + 2 controllers + Camera + Killzone Shadowfall + FIFA 14) for £489.
They did say however that I'd only get confirmation of that at some point in the next 7 days, so presumably they will allow a week for everyone to claim what they want and allocate as demand.
Still waiting to hear anything from GAME (I have two pre-orders but a buddy is having one of them. Don't worry, no price gouging or flipping here!) but now I've upgraded at least one of my orders I'll probably just stick to the core system there.