Google Accounts will not allow the change of the primary email address to another Google email address. Why? That is unknown.
If you have to change to another Google email address there is only one method to do this and requires that you have Google App email to do so:
1) Go to your Group tab in Manage Domain of Google App email as an admin account.
2) Create a group, which is essentially an email.
3) Add the person who you want to be the primary account into the group for notifications.
4) Use that email for the primary email address change and it should work.
This should only work because the group looks like an email, but it is technically a folder of sorts that re-routes to actual emails.
UPDATE (9/2/11): Since Google App Email's updates, this no longer works - at least, not on newer accounts.
There is a newer article on how to change to another Google App Email address, but it requires much more work than what it used to be.