One thing to keep in mind, you get a large exp bonus whenever you aren't on your highest level job. Because of this, if you do switch classes you want to minimize the amount of time you have two jobs tied for highest as you won't get the bonus on either of them while they're both highest.

Also, if it wasn't clear from the previous posts, the boost items on the mog station are not to promote your class to a job, they're just to boost your level and get you somewhat set up at your new level. As part of the boost, you will automatically complete the class and job quests up to your new level, which will unlock the job as a side effect. I don't recommend using them for your first class unless you want to use it on a class other than the one you typically play so you can abuse the exp bonus mentioned above.