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(When) can halacha with a stated reason be revisited in light of new information?

I saw the following question, asked by David Ziants, on mail-jewish today:

> In Daf Yomi, have just started chapter 7 of Mesechet Pesachim, and the first Mishneh talks about on how the Korban Pesach should be roasted. The Mishneh says only a branch from a pomegranate tree can be used for a spit and not from another tree nor from metal. The Gemara explains:
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> 1) Metal conducts the heat so the roasting would be partially from the heat of the metal skewer and not fully from the fire.
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> 2) Branches from trees other from the pomegranate can have moisture and so there might be a little bit of boiling from this moisture and therefore would not be fully cooked by roasting.
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> With today's advances in material production, would it be permitted to use a skewer made of artificial material that does not conduct heat more than wood nor has any moisture issues? Maybe it is even better than a pomegranate tree branch, which is the only option of the Mishneh?

David notes that this is really a more general question in *halacha*: when, if ever, can we re-open a rabbinic decision based on new possibilities that could have changed the outcome if known?  If the rabbinic literature *gives* a reason, and we know something that affects that reason, when can we apply that knowledge?

I once asked a similar question about *yom tov sheini* -- the reason we have it in the diaspora is because of calendar uncertainty caused by transmission delays (messengers are slow).  Today, however, we know precisely when the new moon is, have a calendar that's been set in advance in any case, and have near-instant worldwide communication.  The reason doesn't apply, yet we still keep two days.  The rabbi I asked (I no longer remember who it was) said, first, we don't have the Sanhedrin so we can't overrule, and second, why should we presume there was only one reason?

While we don't have the Sanhedrin today, we nonetheless have a continuing interpretative tradition.  Our rabbis have to make rulings all the time about things that weren't known in history -- electricity, the *kashrut* of new-world birds, new medical procedures, and so on.  Is it that we can cover topics not previously covered but can't revisit past ones?  Is it something more nuanced?

While this question is prompted by the spit for the *korban pesach*, my question is about the *halachic* process, specifically about cases where a reason was given for the ruling and we now know something that would (or could) affect that reason were the question being asked today.

I ask this question out of curiosity about the theory, and not with any specific goals of application.