I see it as a roleplaying choice only. If you impose mechanics, it will probably feel awkward for the player. The best way to do it is have the character freely lean towards Dark Side. But if you would want a mechanical backing, perhaps it would be best to "punish" the player for making a Dark Side choice if he is playing Jedi.
For example, for every Dark action s/he performs, he will "lose" Experience based on Minor Personal Goal table for severity of action. (If this would bring him down a level, you do not force the character to de-level). This experience must now be made up by the character.
However, if the character were to go Dark Side, he gets ALL THAT EXPERIENCE BACK.
What this does, is it grows temptation to abandon the Jedi path with each Dark act. So the more Dark Side actions the character performs, the greater the temptation (for power) to go Dark Side completely.
Of course you'd have to go through and change up Dark Side / Light Side powers (perhaps take away the ranks in Light Side powers and freely re-distribute them into Dark Side powers at the moment of transition.)
That's my thought!
