To reproduce:

1. Download and open attached "repro913928.zip" project.
2. Enable Shaded Wireframe rendering mode in Scene view.
3. Open Sprite Packer window and click Pack.
4. Drag the "Sign_002_amb" sprite into the scene.
5. Notice how the sprite mesh looks in wireframe mode.
6. Click the "Tools/Switch sprite mesh processing on selected texture" menu item.
7. Notice console's notification that Sprite postprocessing was turned ON.
8. Right-click on the "Sign_002_amb" sprite in Project window and click Reimport.
9. Notice that sprite was replaced with hexagon-like shape.
10. Enter Play mode.

Expected result: Sprite mesh keeps it new mesh when In Play mode. Sprite Packer repacks it's atlas with new modified sprite mesh.
Actual result: Sprite mesh is reverted to the original one when In Play mode. Sprite packer does not repack it's atlas with modified mesh.

Reproduced with: 2017.2.0a1, 2017.1.0b7, 5.6.1p1, 5.5.3p4, 5.4.5p1.

Note: After Quitting play mode sprite regains it's modified mesh.