I changed my Apple ID recently (so existing account but new ID).
On my iPhone 4 (with latest iOS5) :
Settings > Store > Account > Sign Out.
Sign in with new ID / password
However, when I go to the store to update apps. I'm prompted for the password for the old AppleID.
Looks like the old ID is cached somewhere.
A full reset / restore didn't fix the problem.
Take a look at:
and see if some of the procedures on managing AppleID will help.
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