If loading a game, load your campaign/profile. When you get to the campaign menu, enter the codes below...
Alternately, if you are starting a new game, click on 'new game' and on the very next screen, enter one of the following. If entered correctly, the screen will flash white for a split second.
Once the screen flashes, go to the bonus menu and look for the characters menu. Unlocked skins are listed with on/off toggles. Toggle the unlocked skin you with to set to G1 style to 'on', and in those stages where that character is present, their basic paint scheme will resemble the paint schemes from the original Transformers cartoon.
The model doesn't change, just the skin!
IE: Starscream is still an F-22 Raptor, just with red and blue streaks similar to original Starscream's.
Jazz: Left, Up, Up, Down, Left, Up, Right
Optimus: Up, Up, Down, Down, Right, Right, Right
Starscream: Right, Down, Left, Left, Down, Up, Up
Megatron: Down, Left, Left, Down, Right, Right, Up
So far I can confirm only Starscream. You may have to start a campaign with each side to unlock that side's skins. I only have the Decepticon campaign running on my profile, so i'm guessing that's why Starscream is the only working skin on my game so far.