One of the ways one knows a prophet is false is if he attempts to overturn a halakhah on a permanent basis. (See Rambam, Yesodei haTorah 9:4, Chinukh mitvah #517.) A prophet could be told to make a one-time exception, as Eliyahu did when he offered a sacrificed on Mt Carmel. (Rambam ibid 9:3)
So, if someone were to come claiming to be Elijah, some other fellow is the mashiach, and a whole bunch of halakhah is getting changed through extralegal means, we know he's lying.
However, with the arrival of the messiah (or perhaps somewhat before or after) will be the restoration of the Sanhedrin. One high court can overturn the rulings of an earlier court. According to the Rambam (Hilkhos Mamrim ch. 2), the rules are:
1- Any Sanhedrin can overturn the legal interpretations of an earlier Sanhedrin, including derashos (laws derived by through formal hermeneutics of the Torah's text). So if they understand a Torah law or otherwise preexisting law differently, the law could change to fit their understanding.
2- A Sanhedrin that is greater in wisdom and number than an earlier court can repeal that earlier court's legislation. This speaks to undoing the creation of new law.
(In practice, I would expect a court that was not greater in wisdom and number would not be too willing to exercise the power to second-guess prior legal decisions either. But the power is theirs to wield.)
So, an ideal messianic Sanhedrin, which would be even greater in number and wisdom than Chazal's couts could indeed change much of halakhah from the way our Chazal interpreted and legislated. In regular, halachic process, ways.
Given the above: Statements like the Zohar's that after the mashiach comes we will switch from following Beis Hillel's rulings to beis Shammai's are aggadic, cannot be taken at face value. It could just be voicing an expectation of how the future Sanhedrin will rule, or even more meta... that they keep current rulings, but the new Sanhedrin would conceptually more Shamaitic in outlook. We'll see that they mean when it happens. But the Torah itself cannot change.
By the way... the source for requiring that law be created by legislative process, with any prophetic input to be ignored, is derived from the verse: "לֹא בַשָּׁמַיִם הִואֿ -- it is not in heaven." (Devarim 30:12) This is invoked in Bava Metzia 59b. Which is why, when Moshe died and 300 laws were forgotten, another 700 thrown into doubt, the rulings are not restored by prophecy. Rather, Osniel ben Kenaz reestablished them deductively, using the rules of legislation. (Yuma 80a)
