Answering the question directly:
Yes, you can experience all past dungeons and quests (seasonal quests may or may not reappear).
(as of 2.0. But no one should expect 1.0 stuff as it's basically considered a different game. That's why devs consider the game to be a year old even though 1.0 came out long before that).
Of course there will be changes. As with many MMOs, bugs are fixed, sceneries changed, nerfs happened, certain mobs may be moved, certain class/job skills might have been altered. But the general content itself is still available.
