Technically they're 16-bit (SNES) sprites not 8-bit (NES), 8-bit ones have a far more limited colour palette. But I'm just being pedantic.

As for the OP, technically they're only for Jobs though, none of the classes have those sprites. Also, as fun to have as it is, how the current class/Job icons in the interface are depicted actually serve a secondary prupose for newer players that isn't readily apparent - and that's to do with the Party list. In the Party List each person's class icon is rendered in a specific colour corresponding to what role they play in the party, viz: green for healing classes/Jobs, blue for tanks and red for DPS. Disciples of the Hand and Land though are just rendered in grey (it seems at one time SE were also planning a fourth unnamed role for in early pics released of the Alpha version of ARR showed ARC's class icon in the party list rendered in yellow, for... ranged DPS? Support?... only for this to be apparently changed early on and ARC's class icon was rendered in red for DPS.).

Either way by changing the class icons to those Job sprites would mean this simply would not be possible. So it would be a cute and fun thing to have, but sadly due to this above aspect of the Party List, it means it's not possible sadly.