We don't need to actively edit out all mention of the EU and the previous levels of canon (G-canon, T-canon, etc.), but a user should edit posts to the new, correct terminology if
- the user is editing the post for something else anyway (spelling corrections, tag cleanup, etc.)
- the post is already on the front page (e.g. an old question got a late answer)
- a new question has been asked and marked as a duplicate, and the old question it is a duplicate of uses the old terminology
I think this is a good middle ground to update/correct the terminology while avoiding the work and spam from purging the site of every mention of the EU and old levels of Star Wars canon.
As for the tags, it is not necessary to substitute star-wars-legends for star-wars-eu, since they are synonyms (and the former is the master tag). See this meta post. As I mention in the linked meta question, I don't think it is necessary to edit star-wars-legends into all the old questions about Legends. But if the above criteria are met (e.g. a user is already editing a Legends question) we might as well add the tag.
I am one of the users (perhaps the only) making the edits in question. An example is my edit to a question about R2-D2. In this case, I was editing the question to remove the character tags (the issue of character tags came up recently in chat, and we have an existing, highly upvoted answer on meta arguing against the use of almost all character tags). When editing posts, I proofread the text and make any other edits that seem necessary. In this example and other cases, I updated the terminology and added the relevant star-wars-legends, which did not exist at the time the question was added. The question specifically allows a level of canon (in the old terminology) which has been deemed Legends, so the tag is relevant.
It's probably also worth mentioning that when star-wars-legends was first created, I did specifically edit a few questions just to add it. This was because users must have score >= 5 in a tag to approve synonyms to it, and no users had such a score since star-wars-legends was new. By making these edits, I was able to get approval to make the old star-wars-eu its synonym. Such edits are no longer necessary.