Change AppleID to @me.com email address
The key is to change the main Apple ID email address or add an alternate email address instead of creating a new Apple ID altogether.

If you logon to appleid.apple.com, you can change your primary email address. There really aren't many big repercussions, and as long as you don't create a new Apple ID you will have access to all of your previously purchased content.

If you have friends that iMessage your @gmail.com email address you will need to add that email as an alternate email address - something you can do from  appleid.apple.com. 

What is interesting is that you can use your @me.com email address right now to sign into your devices. It is added as an alternate email address automatically. When you log in to appleid.apple.com you will notice that under your primary email address is an "Alternate Apple ID". This is probably your @me.com or @icloud.com email address. So, you can use that email address and your current Apple ID password to sign in to your Apple ID - either will work.

Note that you can't login to your Apple ID with your "Alternate Email Addresses", but you can iMessage from your "Alternate Email addresses".