This can be done through eventing, if I understand you properly. What you have to do is store the actor's stats in variables before switching class, then immediately after switching class, use the Change Parameter command to set the parameters to 0, then to change the actor's stats to what they were before the switch by increasing them, because for some stupid reason you can only increase and decrease parameters.
I'll use MaxHP as an example; you'd have to do this for each of the stats (Level, Exp, MaxHP, MaxMP, HP, MP, Attack, Defense, Magic, Magic Defense, Agility and Luck).
@>Variable Operation: [0001:Storage1] = [Erik]'s Maximum HP # Do this for each stat, each with their own storage variable.
@>Change Class: [Erik], [Monk] # The class changing.
@>Change Parameter: [Erik], Maximum HP - 9999 # Do this for each stat, resetting the parameter to 0.
@>Change Parameter: [Erik], Maximum HP + Variable [0001:Storage1] # Do this for each stat, adding the old parameter to the new one via the corresponding variable.