I really regret how a simple Ramban turned into so much more, so please can we get back to meat and potatoes? Tosfos (Kesubos 3b d"h takanah derabbanan) makes an interesting distinction between modifying a gezeirah and uprooting a gezeirah. The gemara is unwilling to accept permanently doing away with the takanah of getting married on Wednesday even in response to a situation of duress, but is willing to tolerate moving the wedding to a different day on a temporary basis. This Tosfos has always bothered me. I imagine the problem with uprooting a takanah is "ain bais din yachol levateil divrei bais din chaveiro", one court does not have legal authority to overrule enactments of an earlier court (with certain exceptions). If so, wouldn't even changing the wedding date also be subject to the same problem? The gemara indicates elsewhere (Megillah 2a) with respect to the Purim that even a change of date is viewed as impinging on the original takanah and the rule of "ain bais din..." applies.
(As an aside: see Tos Meg 5b d"h bikeish which you have to fit with that gemara on 2a).
(Double aside: it feels good to get back on the right track again).