Actually, Godbert may be another good example of an NPC class changing. When he goes into "work-mode," (see above video) he strips down to nothing but his hammer and raw determination, and makes miracles. It even uses the same sound and visual effect as when we change jobs.

There is also Edda. While it's never put on paper or outright stated, everything points to her switching from CNJ to Necromancer...

Lastly, if I recall correctly, Cocobusi, baby brother of the Thaumaturgy guild leaders, took up culinary work as a side job after the story. I can't confirm it as Cocobuki has different dialogue now that I'm deep into Black Mage.