Impact of selecting your level 30 class change

[HyoMinPark]

I’m guessing that you are talking about losing access to multiple role actions (e.g., Protect, Break, Drain, etc.)?

As you level, you will gain access to more role action slots. Right now, it’s just limited to 5 (each has it’s own level requirement you have to meet—at level 30, you should be able to equip 2 role actions, with your third becoming available at level 32, fourth at around level 38, and fifth at level 46/47), but that will change when the new patch comes out next Tuesday, and you can equip all 10 of them if you so please. I do think they will still have level requirements for unlocking the slots, but you level quickly, so don’t worry about that too much.

As you level, you will also learn new abilities for both of these jobs that are specific to them; some abilities will be upgraded to stronger versions of themselves: CNJ/WHM’s Stone and Aero are examples of these—Stone becomes Stone II, Stone III in your 50s, and then Stone IV at level 64, growing stronger each time; Aero eventually becomes Aero II. You will be able to learn new abilities every 5 levels until level 50; after that, it’s every 2 levels. You will need to do your job quests to unlock these though, so make sure to stay on top of them.

[Sateist]

I’m guessing that you are talking about losing access to multiple role actions (e.g., Protect, Break, Drain, etc.)? .

That is correct! Playing Conjurer, I felt that I lost a lot of good useful role actions, where as for Archer, I didn't mind losing them.
This struck me as a quite big difference in class abilities, at the moment you switch to your new job at 30.