So I'm about to finish the game (just in time for Uncharted, hah), and my characters all have reached level 30, so I decided to play around with the class change mechanics.
Most of the time it's not really worth it. The four skill slots I have to play with (2 + one from my main class and one from my new class) just isn't enough to cover most of my old classes abilities and equipment satisfactorily. Another big issue is that I've found that HIT is actually level dependent = all my characters who class changed lost HIT and whiffed everything in the last dungeon, ugh. I had to grind them back up to their 20s to get them to even hit an enemy again.
That being said, I did class changes with three of my characters, and HIT issue aside they're damn fun.
My Samurai turned Ninja... I found that class has most synergy since they mostly equip the same things (I just put on Dual Wield, Slash 1, Slash 2 and Carnage Front). He's absolutely beastly, solving one of the samurai's biggest weakness - a safe single hit damage attack. Assassination when hidden does impressive damage, and Carnage Front with the Ninja's natural dodge ability is an amazing thing to behold.
My Fighter turned Knight... again, good dual class synergy. No downsides or losses since everything a Fighter can equip, so can a Knight. Slapping on Berserk and Genocide means she's equal in sheer power, but with the added defense boost, immunity to paralysis and back row protection of the Knight.
My Ranger turned Fighter... not bad, but I did have to sacrifice quite a bit of Ranger skills. But it's worth it for Ranger's permanent concentration ability + berserk every round. She's already my damage powerhouse, and now she can single-handedly end most lineage bosses on her own assuming others guard her from attacks.
Having these three in action, when the Death Dragon came back to block the magic stone in Undead Town, I killed it in TWO ROUNDS, lol.
Anyway, guess I'm ready to finish the game, play a bit of the bonus dungeon before putting this title down. No regrets, it's a great game