Hellhawk said:......
And why on Earth would I want to re-train as a Kinetic Grimli if I'm a DESTROYER???....
I can't speak specifically to your example but the TechnoMage I currently play is scheduled to become a Destroyer then a Pensive Croanati (in my mental outline of how he will advance and grow given what I know of the setting Im playing in). The path I see unfolding before him is one in which he sees the massive destruction he is potentialy capable of unleashing and flees to the more structured less aggressive prestige class.
There are many in-game reasons why a person might be a Destroyer and re-train as a Grimli.... quite frankly I wouldn't want to just be remembered as a massive blast-canon and that's what being a Destroyer sort of makes you. There are fewer mechanic reasons to make that kind of switch but those decisions shouldn't be based on mechanics... they should reflect the nature and growth of the character.