I just got caught by this too. The same-day flight change policy as stated on CO's website makes no mention whatsoever of the requirement that the same fare class be available, so I expected to be able to make the change (I saw there was availability) and arranged my ride to the airport accordingly.

The website is very misleading. I should have done my research on flyertalk rather than continental.com

In contrast, the current weather waivers for free changes are quite clear on the website that the same fare class must be available. To me this reinforced further that there must be no fare class requirement on the same day change because it was not mentioned.