If you use Transfer mirror the only thing that changes is your cape and job. Everything remains the same, so don't worry about losing stats or skills. As for skill if you did JS your js skill will change to that of the new job, easy peazy.

Mage gets INT jobs. Melee get STR jobs just like you had before.... SO... if you happen to be a mage knight then i suggest you not change if you want to continue with that cause then only thing on the list is likely to be INT jobs and won't get Knight or STR jobs again.. but i dunno 100% for sure.