It should work, as the New HDD should be capable of running at the lower transfer speed. I have an ATA100 MOBO, and don't even know off the top of my head wether my HDD are 100 or 133.
I would however make a point or asking this question, ideally in writing, at the time of purchase. That way you can return it if it does not work.
after all you had said 'i wont in to work on a ATA100 MOBO, and you said here it would, and it didn't!'
What drive are ytou looking at?