Help change retrainer's job.

[Gemina]

If I click "reset retainer class" I'm scared if it turn 1lvl. I want it stay 70lvl

It won't stay at level 70. I also don't believe this information is retained. So if you change the retainers class, and want to change back you're SoL. It would very likely be in your best interest to purchase another retainer unless your current retainer brings little value to you, but this is difficult to fathom if the retainer is level 70 and geared.