You CAN CREATE an icloud.com AppleID anytime and use this for all things Apple, incl. as your primary email.
You CAN CHANGE an existing AppleID not already related to Apple to any other domain.
You CANNOT CHANGE to or from an existing AppleID related to an Apple domain.
Also merging past purchases of 2 or more AppleIDs is impossible, probably due to licensing restrictions from content providers.
That's about as concise as I can put it. Hope it helps to clarify.
What I did to not loose past purchases from multiple AppleIDs (across multiple countries) is to use iTunes Match in my newly created Apple domain AppleID and rematch previously purchased albums & songs. It's not quite the same as merging, but it's as close as the content provider legal eagles let you get.
There are also solutions for Movies & TV Shows but that's outside the legal framework. Google is your friend.
Now last but not least, are Apps! If you have purchased them with an Apple domain ID, then just make that your primary and use aliases for email (or not).
If most of your Apps purchases were done with a NON Apple domain ID, then you have two choices, a) you live with 2 Apple IDs, one for App store purchases and one for iCloud or b) your make your new iCloud.com AppleID the new master and buy the Apps all over again.
For a more in-depth explanation and work-around, read this article:
http://moviesandtech.com/software/looking-to-merge-your-apple-ids-well-heres-the -way-around-it/279