is there an advantage/disadvantage of changing to 2nd job sooner or later?

i remember those ragnarok days that i can get extra skill & stats if i don't change job as early as lvl 40 & finish it @ lvl 50...

does it apply in here? or i can jus change 2nd job @ lvl 30...

As far as I understand things , the advantage is to be able to use cross class skills to help you level...

For example... I took a THM to level 10 to get Thunder I dot so I can use it to level my CNJ, and did my ARC to level 15 but that was just to get ready for the WHM job and no real need for a cross class skill there.

It seems to depend if you can use a cross class skill earlier or not, that will decide if you want to start the new class now or later.

You can change at lvl 30 and there is no advantage to waiting to get your job crystal. The sooner the better actually as with your new job unlocked you will have access to stronger skills or skills that allow you to perform your role far better than if you had remained your base class.

Changing jobs in this game is not a permanent change. When you unlock a job, you get a stone you equip to change to that job. The new jobs are less like a new class, and more like a piece of armor. For the most part, when you equip a job stone you don't stop being the class you were. Your level stays the same (And in fact getting xp as a job raises the level of the class), you can still do the hunting log, most of your gear will stay the same...

Oh, and while there's situational reasons to use classes, for the vast majority of cases it's better to have the job.

As an old RO player, I can confidently say there is no penalty to upgrading your class early in FFXIV. Of course, you will want certain cross-class skills depending on your role, but you can take your time with them if you want.

You can freely change classes at any time, although I will admit having some skills like provoke, invigorate, and swiftcast make the game considerably easier.