Come to the dark side...

[Canaan]

I'm not sure this exactly fits what you are looking for, but in the Unearthed Arcana there is a rule set for something called "Taint" that might be appropriate.

 

[Nyeshet]

Doesn't the same book have rules for instant conversions? I recall an example of an evil guy choosing at the last moment to do a good act, even though it kills him--and that one good act changing his alignment from Evil to Good.

It's Darth Vader at the end of Jedi, when he grabs and chucks the Emperor.

Actually, I think it restored him to Neutral. One deed, after all, is rarely enough to enact a complete alignment change. And while he did save a life, the fact that it was his son's might constrew enough self-interest to prevent a whole-sale slide into Good. Then there is the fact that to become a Sith Lord you have to be a Sith Apprentice then slay the current Sith Lord. While his motives were atrocious (from a Sith's perspective), he did fulfill the letter of the implicit contract by becoming a Sith Apprentice. (And I've always seen Darth Vader as LE with LN leanings, anyway - at least until the end when he effectively became LN. Perhaps had he lived he could have proven he had kicked the 'evil' habit. Having died, however, it is unknown if he would have remained LN or would have revealed himself to be LN (LE) in the future.)

As for using Diplomacy to change another's AL, I would make it harder than that. I would require at least a check a day for a few months. Perhaps every day the check is made and the potential convert rolls a Will save (since, even if he is willing, this is acting against his prior nature, so it should be difficult).

Perhaps every day he fails his save he gains a -1 to his following saves until he succeeds in a save. Once he has failed by at least 20 points every day for twenty days his alignment changes. So if he succeeds off and on, then it may take months to change, but if he is remarkably low willed then it will still take at least three weeks.

 

[Kae'Yoss]

Doesn't the same book have rules for instant conversions? I recall an example of an evil guy choosing at the last moment to do a good act, even though it kills him--and that one good act changing his alignment from Evil to Good.

It's Darth Vader at the end of Jedi, when he grabs and chucks the Emperor.

So he might have slaughtered children, people by the planet, close friends, all that, and then he saves his son and is suddenly a saint? I don't know.

I'm not sure this exactly fits what you are looking for, but in the Unearthed Arcana there is a rule set for something called "Taint" that might be appropriate.

The good old Shadowlands Taint. Could work for this. Especially if you include all the fun parts of being tainted, like internal limbs and other gross deformities.