maybe I'm missing something, but the top part is level and it's the pan inside that's slipped, no? maybe I'm misunderstanding what it is you want to do. are you trying to extend your wall tile to the skipped area rather than to the top of the pan? are you planning on tiling the pan?

sloped, not slipped

sorry if it is not clear, I'm finding it difficult to explain. the floor underneath the pan is level so the pan is higher on one end than the other, making the vertical space available for tiling on the window wall different. my question is, is there a way to mitigate the difference in height with tiles, or will we have to pull everything up and make the floor under the pan sloped, or?

Leslie:

You are misreading your drawing. In the section view, note that the lines on the right at 4 1/2" indicate the top of the pan to the foot. The 3 1/4" on the left is from the bottom of the top of the pan to the foot. The floor, the feet, and the top of the pan are level; the drainage is built into the pan.

yes, what @Joseph Corlett said. you're misreading the diagram.

Yes I agree with the others, you are misreading. Just looking at your picture, the top rim of the pan looks very level. Its the inside of the pan that slopes and it has to. No, you do not need to pull it up, you cant have an unlevel floor underneath it. Your tiles will be just fine