Not sure I quite followed but if you are asking if I have completed quests that I did before 2.0 and I turn them in after 2.0 will I get XP for them.
Again I can only answer that on PTS this was the case. I see no reason to believe this will not be the case. If I recall testing that some people had done with Character transfers, with XP boost got them 1.2 - 2 levels for a full set of Bel, Section X, and Ilum Dalies.
That being said, I don't expect the grind from 50 - 55 to be too painful. People will be running the new planetary quests and running things like EV HM for gear. I suspect most people will cap out at 55 before they finish new planet.
While I never played WOW, I have heard from some people who did that some people tried this once when the level cap rose, and gained a level or two immediately. Afterwards, apparently Blizzard stepped in saying "no no no" and rolled back those levels.
While I don't think having a couple quests to turn in immediately when 2.0 drops would be gamebreaking due to the limit on completed quests you can carry, it would be nice to know the answer to the question so no one wastes their time trying that.