Nope, you can switch back to PT1 at any time and do any quests you haven't already completed.
The only thing that's a one-shot deal is when you finish PT2, it becomes "PT2.5" and all enemies in the game level up to ~50. You can't go back to the original PT2 where the enemies in the starting zone were ~36. But the new level scaling scheme being introduced in the October patch may change how all of that works anyway.
The only thing that's a one-shot deal is when you finish PT2, it becomes "PT2.5" and all enemies in the game level up to ~50. You can't go back to the original PT2 where the enemies in the starting zone were ~36. But the new level scaling scheme being introduced in the October patch may change how all of that works anyway.