How to promote to Samurai with Boost

[ItMe]

I believe I created a lvl 7 warrior. Then boosted to DRK long time ago. My retainer has the option to become a warrior, but since i am only lvl 7 with it, her max level is 7.

Then, unfortunately, you'll need to level Warrior a bit more so your retainer can level some more and reach the point where they can swap to Sam.