So I'm aware of this whole class 'upgrade' as it were. For instance, pugilists 'upgrade' to monks at level 30 pugilist and 15 lancer. All well and fine.

Do you 'have' to upgrade? For instance, archers become bards, can they stay archers or is this not viable?

Then there's the shiny new rogues, do they have to become ninja? So on, so forth.

The next question is the '15' portion of these upgrades. I would assume there's at least some benefit in leveling up the other side of these jobs?