My understanding is that's correct. The full Ursaphant Rider tier, while it has the same benefits as Early Bird Ursaphant Rider, does cost $25 more. You simply got the same benefits for slightly cheaper due to pledging in time to claim one of the early bird slots. Since not all tiers had early bird slots (and it would be too late to claim them anyway) any upgrades will require you to pay the full difference between your current tier and the one you're looking to upgrade to. For example, to reach Baron, your total amount pledged would have to meet $250, just like all the other Barons (you couldn't get it for $25 cheaper simply because you upgraded rather than paying for it all-at-once.)
EDIT: If it appears to let you upgrade from Early Bird Ursaphant Rider to full Ursaphant Rider, though, essentially offering to sell you nothing for $25... that does seem a bit strange. I think that any tier you already have access to through early bird offers should probably be grayed out just like the others.