I guess you could just copy the actor information and create two new actors where the stats are the same (or changed depending on if you want the stats to change when the character is changing class). So you set a cap of example level 25 for the first actor, and then you make a duplicate of the actor and raise the level cap to whatever you want it to be. Then on the plot point all you have to do is switch out the first actor to the corresponding actor for each of the 2 new classes.
Just make a choise option and make the event switch out the actor for the new class, set the new actor to start at level 25 without any starting gear (or with) and set the cap to 50. I guess you can change the look of your character this way as well if you want him to look different when he's a new class.
If you want the player to be able to switch between the classes whenever he wants, all you have to do is setting up this in the event:
First off, a conditional branch that will trigger depending on which class (actor) the player is currently using.
Let's say we have Guardian as the current class and want to switch to Berserker.
We then add a conditional branch that will only trigger if the player is using the Guardian actor, and add this:
change party member: Add [berserker actor]
control variables: [001:set Guardian Actors exp value] = [Guardian actors] exp
change Exp: [berserker actor] + [001:set Guardian Actors exp value]
This will make sure that the exp and level will always be the same even if the player decides to switch to another class.
I'm not sure, but that's the only thing I can think off without using any script and I think it should work pretty fine
Edit: The things that could mess this up is, if you want to keep the original stats from your first actor and have the new class distributing the stats in a different way after the change. It's possible to use the same setup as we did with the exp value for each of the parameters, but that would mean some more work in setting up all the variables, and I'm pretty sure there is a easier way to do it.
It's a valid way to do it, but that's all I can think off without the use of a script, and I'm pretty sure there are other (maybe easier) ways to set it up as well.