Just my opinion, but I think it depends on how many readers you have.
Correcting actual errors in grammar or spelling is always useful. The same goes for other errors: I once almost missed a line where I attributed a line of speech to the wrong character; e.g. "John said" instead of "Alice said".
Another type of error might be correcting some big logic hole in your story, if you can do it without a major rewrite. Like a character knowing something in chapter 15 and then not knowing the same thing in chapter 17 (perhaps due to rewriting in C15 after C17 had already been written).
If you don't have many readers, the extending the story or changing the plot or characterizations might be fine.
If you have lots of readers, I would not touch that. Like ggiaquin says, it might change the story or characters for the worse. Further it could produce a situation in which somebody recommends your book, and the new reader finds it worse, it drags, it is boring, it takes too long to get to the point.
I admit I am not familiar with online publishing or ebooks, so I don't know what the expectations are in that market.
The exception I can think of, which does not apply to a single year, would be cultural updating, to make a book acceptable to a new generation. A book intended to be "happening today" that was written before the advent of cellphones, the Internet, sexting, ubiquitous porn, acceptability of homosexuality and homosexual marriage, and a hundred other cultural developments in the last thirty years might still have a salvageable plot and characters, and the author could provide a new edition for that purpose.
Or even with more recent developments: A fictional novel finished in early 2015 that had a significant political component might be in need of an update already, to reflect recent political developments in the USA, even if the political component did not extend to the national level. Assumptions about the gravity of office and what candidates can and cannot do, and office holders would and would not do, have been broken, so the fiction might need some propping up if it depended on such assumptions, in order to continue to be plausible.
On the idea of "extending" scenes, I think that is generally a bad idea. If you did not need the material to tell the story in the first place, why is it needed now? What makes it NOT a waste of time and reading, if it has no real impact on the story?