Need Help - Retainers - Do they retain experience already gained

[Nihility]

It's just an aesthetic thing so you can equip your retainer with job specific gear.
In the menus it will probably simply be like a promote to job button that does nothing but allow them to wear the job gear

[Noxifer]

In the menus it will probably simply be like a promote to job button that does nothing but allow them to wear the job gear

Nope. That's not how it's going to work, judging by this line in the patch notes:

* You must reset your retainer's job before assigning a different class or job.