How to transform flat face to concave one - model palate? I'm relative new to blender.

I'm trying to model palate in blender. So far I managed to create flat face between gingivas as in picture below:



Now I'm trying to curve it to be more realistic, result is expected as:



So far I tried with lattice, but it's giving very angular edges. 

How do I bend this face to be concave?
Isolate the ground plane (or remove it and create a new one. Then subdivide it and use Proportional Editing (Keyboard O in edit mode) to shape it to your liking.
EDIT: here the illustration: Youtube - Blender teeth modeling. It's just one way though. There might be better ways depending on the existing topology and what kind of quality you are trying to achieve.