Question About Classes

[Dark_Legacy]

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

While you do not have to upgrade, you will miss out on the huge stat boost you get from equipping your job stone and you also miss out on the 5 job specific abilities available to each job. for example this is what bard gets and if you stay archer you won't have access to the 5 actions listed, also groups may shun you, while not so bad at first once level 50 people may start to boot you from everything.

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?

Yes their is a benefit, you get access to what they call "cross-class abilities" each job gets access to certain cross class abilities, but only if that certain job has been leveled to the required level, in the link above it also shows you what cross class abilities are available for bard and what the required level of the original job is needed to obtain them.