I don't know. I've been @mac.com and @me.com all along. I've never tried that transition, so I don't know how much trouble it would be. I understand that you can't change your existing iCloud account from a firstname.lastname@example.org AppleID to an @me.com AppleID, so you'll have to set up a new iCloud account under a new @me/@icloud AppleID.
If the fragmentation you noted bothers you, as it seems to do, you can avoid it (or at least reduce it) by using a single iCloud account for everything. (You can still keep the older @gmail.com and/or @me.com email addresses if people know you by one or the other of them.)
Thanks for bearing with me, markwmsn. I've used a PC for over 25 years but have only been a Mac user for 2 years and never had an @me.com address. Accordingly all of my email comes to Gmail YAHOO or my ISP, Roadrunner. I had no complaints re: Mail until I just upgraded from Snow Leopard to Mountain Lion. My Roadrunner account was (and still is) my primary email account and this was the only account whose messages appeared in my Apple Mail app . Thinking it was a neat thing to be able to get all mail through Apple Mail, I linked all of my other email accounts. It's been a nightmare! Sent messages would be duplicated as many as 10 times in the Sent folder. Deleting them from Trash worked momentarily and then they popped up again.
I might add that these same accounts have been linked to my iPad for 5 months and they all sync properly with each other. As a result of all of these problems, I just unlinked everything but the Roadrunner and iCloud accounts from Apple Mail, although I don't even use the iCloud account.
I've been a staunch Apple supporter to my friends but this Mail and iCloud experience has made me rethink the idea that "everything just works on a Mac,"
Thanks for listening.
I'm sorry you've had such a problem with Apple's Mail application. I've been a Mac user since the year they were first introduced, plus a Windows user to a lesser degree (in emulation and directly on the hardware).
I have ten mail accounts attached to Mail on the machine I'm using at the moment, at least half of them active on a daily basis. They are on iCloud, Gmail, Yahoo, ISP, and ESP servers; most are IMAP, a couple still on POP. Most are also connected to my iPad.
I've never seen the duplication of sent messages. I do see the occasional duplication of incoming mail, which I put down to an oddity of how I use mail rules. I also see an occasional message "come back from the dead" after I've trashed it; I think that's due to a problem in communicating with IMAP servers.
If you want to try for help diagnosing your Mail problems, you should post a separate message on that topic. Somebody will probably be able to give you some assistance. I'm not here every day, but I'll try to jump in if I notice the thread.