Just to throw my 2 cents into this conversation.
There is a catch with changing things around retroactively: People who previously did not see a value in opting for the premium package, might suddenly feel that they are missing out on something they would want, that simply wasn't offered to them before. This would result in those players feeling let down, betrayed even, which would only be worse if anything game changing (map tiles, soldier art, etc) would be offered, as it would inherently devalue their previous "purchase".
I really do not envy the goldhawk guys on this one, as this is most certainly a tough nut to crack.
My advice would be this: whatever you offer, make sure it has nothing to do with the actual game. No tilesets, no faces, no flags, no fancy loading screen.. etc. Doing this would just - even if done with the best of intentions - would simply just backfire.
Additionally, you might want to offer people who previously opted for the standard preorder, the possibility to upgrade to premium, giving them the chance to change their choice in light of what the premium will contain. Again a double edged sword as you would have to make sure its not too tempting, as then people would be screaming about how they are "coerced" into upgrading.
If there is one constant about players, it is that we can never be completely satisfied, no matter what you do, we will always find the small dirty spot on the shining white suit to complain about .
So be very careful with retroactive changes as those have the potential to severely mess things up on the PR front. Oh and YAAY for steam, and YAAY for beta. I'm just waiting for the beta fog to clear up a little before I make my final approach.