To change to a 3rd class, one must be level 99 of either the 2nd base class, or 2nd transcendent class. Changing to 3rd job is not like transcending, and you will remain level 99 when you change.
If you chose to change to 3rd job before transcending, you will be unable to transcend in the future. The primary advantage of transcending first is that one is able to learn the transcended skills before turning to the 3rd class, whereas changing immediately from base 2nd class would never be able to obtain them. Additionally, characters that change from transcend will maintain their 25% hp/sp bonus over non-transcend characters.
Once you change to a 3rd job character you will be able to level past level 99. The level cap for 3rd class characters is base level 150 and job level 50. 3rd class characters are able to increase status points above 99, to a maximum of 120 points in each stat. All 3rd class characters are able to use equips from previous jobs, including transcendent equips, no matter if the previous job was 2nd class base or trans.
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