I thought it was pretty clear. Here's the screenshot of the dialog box you get when you click the update button. The link to more about updating explains the advanced options and keeping old versions.
I had no issues, everything moved over to the new version.
As I said, looking back, sure it's pretty clear. However I did it in a hurry, expecting it to be the same as always before. My mistake. The dialog box for me had a much lighter text, not sure why, however I blew right over it.
As to Mark's question, no all of my plugins did not cross over, photokit, in particular, as it came up telling me the product was already licensed. An issue I have had before, and after getting it fixed in CC, just have left it there. Topaz asked for licensing info, which I had, Nik worked fine. Neat image asked for licence info, I had to dig up. Focus Magic, came up. Extension Manger in 2015 kept giving me errors, when I tried to use a few odd extensions I still use for stacking, but the main stack workflow is OK.
Again I am a creature of habit, and learned a lesson. However several folks I know also did what I did and paid the same price. You get into a routine with Creative cloud, just click on update and go, at least that's the way it's been since CC, and never once did it attempt a older version removal.
One other issues I believe, if you allowed it to blow away the older versions, and install 2015, you can't get them back via Creative Cloud at least I couldn't. It will only show CS6 and 2015. So again if you need those versions be careful. I was able to get them back as I had a 1 month old image of my drive. But as no CD's ever ship since CC, other folks may not be able to get back as quick if they want to.
I might have caught the text box if the wording had been in red or bold.
Remembering some of the issues (photomerge mainly) 2014 CC first had, I would not just want to be running the latest version.
Paul
