the easiest way would be to create one movieclip that has
1. a rectangular mask (to mask everything),
2. a dynamic textfield with whatever colored and embedded font.
3. whatever colored background
4. another textfield that has a y property greater than the mask so it's not visible until the mask is rolled over. embed the font
5. another background for the rolled over movieclip
everything will need instance names so you can control with actionscript. you can then assign rollover and rollout code to your movieclip mask.
on rollover, you'll tween the first textfield (and possibly background) to a y less than your mask. tween the textfield with easing. and you'll tween the 2nd textfield and 2nd background to about the same y as your mask. again, tween the textfield with easing but not the background.