Q: Upgrading my RAM has increased onboard Intel HD3000 VRAM, which GPU will mavericks choose to use?

Hard to say. Probably the discrete because it's more powerful despite the lower VRAM.

Get the GFx Card Status app.

http://gfx.io/

It will show/notify of GPU changes and may even improve your battery life.

I thought the rules for switching to discrete would be the same, the only difference being how much RAM the integrated gets when it is active, but I have no proof that is how it works.

The only source of info I know of doesn't go into that much detail:

About integrated video on Intel-based Macs (Apple)

it uses the discrete (AMD), But i use Photoshop and i need the AMD's VRAM to upgrade to 512mb to enable Photoshop 3D features. Any help please