My opinion is that minor updates to an existing hub are good. It keeps it fresh and current.
However, if you are adding a lot of new material, I suggest doing a new hub. With a new hub you will have to rewrite your old material because you are not allowed to have duplicate material on HubPages. Rewrite it and blend in the new material.
I had the same issue (sort of) with my two most recent hubs. I discussed the different types of world governments and then did a final segment about how Trump was governing. The next day, I decided the Trump stuff really didn't belong at the end of this hub. I removed it and started a new hub. This meant I had to take some of my content from the first hub about democracy and rewrite it so that it could serve as an introduction to my comments about how Trump is eroding democracy.
I ended up with two hubs and I see that they are being paired by HP. When you read one, you see the thumbnail for the other on the right hand side of the page.
I advise you to take this route. Two hubs are better than one and you will gain synergy if they get paired by HP.
I expect that my hub about Trump will require updates as things change. I did try to keep my content as general as possible--his style of governing--so it would be more evergreen.
This was my first attempt writing about politics on HP. I've been thinking about how to do it for months. Politics changes so rapidly that a hub could be out of date in a week. I decided that my political hubs would be "big picture" hubs to avoid having them become dated.