In general, editing the close banner is often good.
This is a tough question. On the one hand, it seems silly to reopen a question that should be closed, and inconsistent with the way closures work in the Stack Exchange system.
Five close votes (or any number if the last one is cast by a moderator) close a question. This is the case for every close reason, and whether or not people choose the same close reason. And if people all choose duplicate but specify different duplicates, that closes it just as easily as if everyone gave the same duplicate.
Multiple "Possible Duplicate" questions may be listed if more than one is voted on. And when close voters choose different close reasons but some choose duplicate, their proposed close reasons typically still appear. (This sheds light on the meaning of Possible in Possible Duplicate.)
So it seems the system is designed to close a question when there are enough people who want to close it, irrespective of why they want it closed. Thus, if a question is already closed and should be closed, but the reason was wrong, it seems it should remain closed, rather than being reopened and re-closed again.
As I understand it, no one (except a moderator) can vote to close the same question more than once. So all new close voters would be needed to re-close it.
Moderators can merge answers from any closed question to any open question. So if a question is closed as a duplicate of the wrong master question, that won't stop it from being properly merged (though it may be helpful, when requesting a merge, to tell a moderator what question it really should be merged into).
For these reasons, I think when there's a clearly better duplicate, editing the automatically generated duplicate banner to display it is a good thing.
But I don't think most of us made a mistake when we closed this.
gertvdijk said, "I must have copy pasted the wrong question in!"
He cast the first close vote. Before I cast the second vote, I looked at the proposed duplicate and saw that it was a well-answered question about what to do for an interrupted release upgrade. The question we were closing was about the computer being shut down during the upgrade, which definitely qualifies as interruption.
Furthermore, this is quite different from the duplicate you've proposed--in the question we closed, the OP rebooted, tried to update, and found he could only perform a partial upgrade. In the duplicate you've changed it to, the Ubuntu system doesn't start at all, and the solution given is to chroot in from a live CD.
- Neither question is exactly the same.
- Both are relevant.
- Both are reasonable choices for duplicate.
- The question you've chosen superficially seems better, but the situation there is actually quite dissimilar.
If I had been looking at both possible master questions when I closed this, I would still probably have picked the one I did.
Usually, adding to the close banner is better than replacing.
I think the best thing to do here and in most situations is to add another possible duplicate to the banner, without removing the one that's already there.
If the one that's there will clearly not help anyone, it should be removed.
But in situations like this--and other situations, too, where it's way wronger than here--it should usually be left in.