There are three Autobots who became Decepticons that I can think of:
1. <PERSON> from the Armada continuity
<PERSON> was once an Autobot in the same military unit as <PERSON>, looking up to his peer due to his own inexperience in battle, and they considered each other good friends. However, that friendship was broken one day when <PERSON> was trapped under rubble in the middle of an intense fire. <PERSON> could not lift the wreckage alone, and left to get some help, promising to return, but he never did. An arrogant Autobot officer forbade Hot Shot from returning, claiming the fire was too intense and they couldn't risk more lives.
Meanwhile, <PERSON> saw a figure approaching through the flames, thinking that at last, <PERSON> had returned. But it was <PERSON> who saved him that day, and in return, <PERSON> became a Decepticon.
<PERSON> hence left his large scarred Autobot symbol (ironically looking like it had been crossed out) on his robot chest / car bonnet and put a new, smaller, Decepticon symbol underneath. This is also shown clearly on the toy:
See also the Transformers wikia page.
1. The Fallen
The character called The Fallen has occurred in multiple Transformer universes. He is typically described as one of the thirteen original Primes who then went on to become the founder of the Decepticons.
For example, in Transformers: Revenge of the Fallen, The Fallen is described as:
... 167
the first Decepticon and the true founder and leader of the Decepticons (as <PERSON> accepts the Fallen as his master), and all future Decepticons would wear an insignia which is similar to the Fallen's face.
See also tfwiki.1
1. And from Transformers: Dark of the Moon (spoilers)
! Sentinel Prime
! ! Once he arrived back at their base, Sentinel ... betrays the Autobots, kills <PERSON>, transports the pillars to Washington D.C. and rendezvous with <PERSON>. After teleporting hundreds of Decepticons to Earth, he has a confrontation with <PERSON>, but spares him, in the hope that he would recognize the moral of what he was doing. He demands the exile of the Autobots, who are apparently destroyed while leaving the planet in their space ship. Sentinel usurps <PERSON> and assumes command of the Decepticons to oversee the final phase of the plan in Chicago.
Caveat:
! Strictly speaking, Sentinel Prime did not actually join the Decepticons (including getting the purple badge) but he did ally himself with them and even assumed command of them (see above quote).
1. And an honorary mention should go to this character from Beast Wars / Beast Machines (spoilers for Beast Machines)2
! <PERSON> (Vehicon) was revealed to be Rhinox (Maximal). Though he was initially reprogrammed by <PERSON> to be a Vehicon, <PERSON> somewhat embraced it for a while (after regaining his <PERSON> memories). Finally, he expressed remorse for his actions thereby reverting, in his spark, to a Maximal.
1 Credit and thanks to <PERSON> for this suggestion.
2 With thanks to phantom42 for this suggestion. 715