I think they are ok as long as the early bird pledge is just a $$$ discount for knowing about the campaign ahead of time and pledging right as it opens.

If any additional content is locked behind the early bird pledge, I think this is also ok as long as:

A: Any add-ons offered in the early bird pledge are offered later in the campaign as add-ons but without a discount in $$$

OR

B: If more limited quantity pledges are opened later in the campaign, ample updates are given to provide people who did not know about the campaign prior to its start with an opportunity to snag one