Could you give us URLs please?

As it is, it sounds like plagiarism, but I can't really tell without reading the stories in question. And just because it *sounds* like plagiarism doesn't necessarily mean that it *is.* It could possibly be a case of someone archiving an earlier version of the story under an old pseud without permission or something. I know of writers who will post stories on their websites and then revise them later, and people who have changed their pseuds. One writer I know heavily expurgated some of her adult stories when she put them on fanfiction.net, so there can be multiple versions of the same story by the same writer out there.

As for what happens, a lot depends on the fandom itself. At the least, I think the "rewrite" should be taken down and the plagiarist warned not to do it again. Generally, the plaigiarist will be persona non grata for quite some time in that fandom as well.

Tiger M