I've bought well over a thousand songs from the iTunes Store over the years. In the time, I've had three email accounts linked to the store. It has not been an issue until the launch of the store's most recent services, "Complete My Album" and "iTunes Plus".
Now, in order to see if I qualify to (a) complete an album, or (b) upgrade my library to Plus, I have to log out and log back into each account. Let me say here that this is not a herculean effort, but it is annoying nonetheless.
I contacted Customer Support to see if there was any way to consolidate the accounts into a single account (my current account is based on my "permanent" .Mac account) but I've been told there's nothing in place to do this, and received a lengthy cut & paste email about how to deauthorize computers.
I would think that Apple would want to make this option available to users -- I spent another packet of money upgrading the albums & songs in the two other accounts that I wouldn't have if I hadn't done some investigating. In other words, this situation is equal to lost revenue.
As a stockholder, I'd like to see this option implemented in some way. Make it a "you can do it once every year" thing, like the deauthorization situation, but make it available.
No need to create a new iTunes account. You can simply update your current account with any new info when you get a new email address, new CC or move.
But it would be a good idea to allow account name changes. Perhaps give the the songs a code for the account instead of the iTunes account name which is an email address. This wway all the songs you purchase would not show the email address you had 5 years ago.