The lattice object/modifier pairing is a under appreciated modeling tool in Blender, which makes it fast and simple to make sweeping changes to high density polygon meshes, in a way much faster than even sculpting.
This quick Blender Tip tutorial video ( available in HD here ), illustrates the process.
Basically, you envelop your existing mesh in a Lattice Object:
Zoomed to illustrate better:
Then in object mode with your mesh selected, go to the modifiers tab and add a Lattice Modifier, click Object and select your Lattice:
Now select the Lattice object, and set the number of divisions you wish ( how many control points in each direction ):
Switch to Edit mode and make whatever alterations you wish, changes to the lattice will be applied to the underlying shape:
When done, with the Mesh selected, go to modifiers and click Apply:
You can now delete the Lattice, and tada, updated polygon mesh:
The video also shows how to set a shape key for animation if you are curious.