In February 2021, Project Euclid made some changes on their website: Project Euclid Platform Migration FAQs.

It is probably a consequence of this that the links starting with http://projecteuclid.org/DPubS do not work anymore. (At least the ones that I have tried.) On MO I get 485 posts when I search for such URLs. If I try search in the whole Stack Exchange network, I get 950 results. (There are also links with projecteuclid.org/download - which I did not check extensively, but at least some of them work.)

To include one specific example, let us take the link http://projecteuclid.org/DPubS?service=UI&version=1.0&verb=Display&handle=euclid.nmj/1118801157 which was present in the revision 1 of this answer. The link does not work - but looking at this part of the URL handle=euclid.nmj/1118801157, you can arrive at a link which actually works: https://projecteuclid.org/euclid.nmj/1118801157. (Although you can see that it was actually replaced by a DOI link in revision 2.)

Of course, it would be better to have links which actually work. At the same time, manually fixing (and bumping) about 500 posts does not seem like something which would be optimal (at least if it can be avoided). Some users have already fixed some of the links of this form here on MathOverflow - this SEDE query returns 44 results at the moment.

So the natural questions are:

1. Can the links be converted to the short format https://projecteuclid.org/HANDLE automatically (and without bumping)?
2. To which extent is this format of the links reliable? (If it is likely that those links stop working at some point, all work done in converting those links would be in vain.)

Concerning the reliability of the link, the FAQ post linked above says: "All URLs on the retired platform redirect to the new platform." So perhaps we could hope that links in this format should be stable.

As far as the first question goes, we should keep in mind that the tool which can be used for fixing links without bumping can only replace a specific string by another string. (Some more advanced tools - such as regular expressions - aren't available at the moment. The things which are available have been discussed a bit in another recent post on this meta.) In the example above, this would mean simply removing the substring DPubS?service=UI&version=1.0&verb=Display&handle=. So the question is whether Project Euclid links used on MO contain only a few such substrings. (Maybe it is worth looking also at other Stack Exchange sites - some of them have lot of such links, too; mainly Mathematics and Cross Validated.)