This process is discussed in more detail here and there are screenshots. Also well described here in Italian. Use a translator if you need to. The Italian version by @rugyada has even better screenshots.
As you can see we have been testing this since OM Lx 4.2 Alpha. However with system upgrades like this it is always possible that some users will experience issues. That is why we continue to suggest preference for fresh installs if at all possible.