Keynote animation to scroll text box content
I just did something similar that may help. I broke the text up into blocks. Each block being the contents of one 'page' I wanted to display. I then Positioned the blocks one above the other in the order I wanted them displayed.

Next I left the first block which would be displayed at 100% opacity and reduced the others to 50%.

Then I added a move to the blocks so that they all moved up the screen the same amount at the same time. 

Next I added an opacity change to each block so that it either reduced to 50% or increased to 100%. 

Finally it was just a matter of adjusting the animations so that the blocks all moved together and changed their opacities at the same time.

The final effect was that as the blocks scrolled up together, the currently displayed block fades out, and the one becoming the current block fades in.

I'm sure that it's probably possible to do something like this using magic move if you want to use multiple pages to generate this effect.