OK, so the whole back-story of this forum post thing is this:
One time i felt like blowing stuff up as scorponok so i went to the desert and did his "chapter" mission thing, then when i was done i was blackout again, as usual, and then i was bored so i felt like letting everyone kill me. (lol) When i revived, i was SCORPONOK! I just discovered the way of how to change the character. i experimented with this on barricade, i went to the big city and i was megatron (this only happens after you beat this level/area) i completed barricade's mission and killed myself as megatron, and i was barricade! To do this for yourself, i'll give you the steps.
1.Do and complete the mission with the character you want to be (example: jazz's mission)
2.After you complete the mission,blow stuff up until the tanks and choppers show up, then let them kill you. (One of the easiest ways to do this is to hit a vehicle and wait until it explodes in your face, that reduces your health.)
3.then you are the transformer you have chosen!
If you still don't get it, just do an effing mission then suicide.
I just wanted to add this on, some people have been complaining about no free roam and not being able to be your desired character, and that a transformer keeps telling you what to do all the time. (example: Jazz, get to the beacon, and cause a distraction)
First of all, if you ignore the annoying transformer telling you what to do long enough, eventually, they will stop. And they wont start over if you die, either.
Second, i already told you how to change your character.
and Third, stop complaining, you can do whatever you want, even retry the missions again.
I think the main complaint about no free roaming isn't so much that you can't walk around outside of missions as that there's nothing much to do in any of the maps; since you can only interact with the environment by destroying it and there's nothing to find that would help you in game, it's kinda pointless.