Currently a lv37 WHM with lv15 ACN, just completed main quest line up to Titan, also received the quest to some Sunken Qarn (??) dungeon.
On my WHM/CNJ, I have accumulated a lot of rested exp, it is in blue color since I was lv35, I think it will still be blue when I hit lv38...

My problem? I plan to level up THM, yes, my 3rd class, for some cross-class skills I am interested in.
Question is: What will happen to those rested-exp I accumulated?

Will it disappeared, forever, once I obtained the new class?
Or it will re-appear when I switch back to CNJ/WHM?

Anyone who tried switching class & leveled up with huge amount of rested-exp? Pleas share your knowledge, thank you.

If I am not wrong, the rested exp you're talking about is the exp that you were afk or log off in a sanctuary and had accumulated.

If you switch your class, the bar (it is counted as % rather than exp) will be changed accordingly. once you depleted it as another job, your other jobs will as well be depleted.
Sorry for the bad english.

So...means:

Now, I am lv37 with rested-exp all the way until lv38.
If I switch to a new level 1 class, and level up to level 2, then BAM all those rested-exp will be considered used-up, therefore vanish?

Your rested exp is shared among all classes. You can get back by just logging off from sanctuary or from inn.