Completely forgot about that.... I think buff may also be correct in that sub-classes like "House keeper" and "Scribe" maybe able to be leveled just by use, without requiring characters to actually level up. Adding a different class tree however like "Enchanter" + "Sorcerer" would have to be done by leveling up I would expect.
I thought the entire point of the show to this point was that there was magic that could change the "Laws" of the world (assuming both elder tale & reality) and the Sage guy was investigating that specific magic class. Personally I think he gave Shiroe enough information to make a world class spell so he could observe the outcome. In fact he just made Shiroe invent a way to make People of the Land immortal.... The Sage is probably up to no good. Of course he could of just facilitated for the sake of observation.