i was thinking this myself, but im not sure.
1- in the new rulebooks after it says about upgrading it will then name the unit E.g x unit for Y points can become Z unit. Z unit has upgrade:.
it deffinatly says that they have a name change and then says what the rule change is, so i would argue that once they are mirkwood guard they are no longer wood elves as it says that that name "mirkwood guard" and then says the stat change"
2- in the latest white dwarf, the uruk player in atofg changed the names of his marauders in his army list, which i think suggests that you do change the unit name.
however ther is nothing in the faq section and ther is nothing in the rulebook that makes it clear, so i would just ask opponents if they don't mind you using both, but i think it does kind of ruin the whole point, as these give character to warbands and this would make 2 loads of elves from different places the same
i think it is really up to you and whoever you play with till gw says anything official or someone on his forum come up with a diffinanitive answer.
hope this helped although im not actually sure i answered your questions...