Many thanks! I somehow missed that thread. We tested an upgrade from 1.0.x to 1.5.5 with a real dataset and couldn’t find any issues so far. So from that point of view it seems to work. But testing is one thing, production another 
I also know MySQL/PostgreSQL and in these cases you definitely don’t want to skip a major release
So as Vault is basically also kind of a database my initial thought was to do the same and upgrade step by step. We use Google Cloud Storage as HA backend btw.
I couldn’t find anything related in the official docs. So currently it seems that if you want to be
you need to upgrade step by step as there might be long term issues which you might not detect during testing and only come to light after a few days of usage - at least that’s one of my fears.