You CAN change your pledge in PM, you just cannot use the early bird discount.
We wanted the Premium to be the same price for all backers, this is why there is no "upgrade" between Standard and Premium, but you can always have the option to change to Premium by contributing the full amount. In the PM you can also add add-ons and more copies of the game.
Let me explain in detail.
Difference between standard (33€) and premium (46€) is 13€. So, in the PM you can not add 13€ to your Standard Early Bird (27€) and get the Premium for 40€. Instead, in the Pledge Manager (PM) you must add additional 6 more euros to get the Premium for 46€. In this way the Premium Box pledge is the same for early and late backers. So, you can change the box, but you can not "upgrade" (take advantage of your early bird).
I think our time based early birds really complicated things :)
Let us know if you have more questions or something is unclear.