MacOS – “This Apple ID is valid but is not an iCloud account”

This article from Apple Support answers some commonly-asked questions related to the Mobile Me to iCloud transition. This article from Apple Support explains how to move to iCloud. From this article we read the following:

Inactive and expired MobileMe accounts do not need to move to iCloud. Simply use your inactive or expired account to sign up for an iCloud account and follow the onscreen instructions

This article lists some additional troubleshooting issues with accounts and iCloud. It is important to note that the number one troubleshooting issue mentioned is iCloud outages. As it has been mentioned on this thread, iCloud is being hit pretty hard today on its official launch day. From the article:

Check System Status to make sure there are no outages or scheduled maintenances currently affecting your iCloud account. System Status is displayed at the iCloud Support web site.

The "Lean More" link on the original question (above) is the same link as the first one I include on this answer. If some of these links are not working in your area, it may be a matter of waiting for the pages to propagate across Apple's worldwide servers (and their backbone providers). The links work within the US. It also worth mentioning that more articles are literally being made available by the hour. In other words, There is a chance the some of these links will not work immediately for you, but they eventually should. Hope this helps.

Here is a fantastic article on moving to iCloud. http://support.apple.com/kb/HT4918

And, yes, you can keep your current @me.com address. Instead of being associated with MobileMe, @me.com email addresses are now iCloud (and MobileMe user will be migrated over), so the email address that comes with iCloud is @me.com. Again, I believe you are able to keep your current @me.com address.

Unfortunately, merging wouldn't be possible.