Sorry I didn’t write back yesterday.
You can’t really convert, you’ll have to delete your old account set up and make a new one. You’ll end up with duplicates just like you thought if you don’t delete the old stuff first.
Back up your .pst file as justin suggested, you can find out where the pst file is from Microsoft.
Then, you can either delete all the messages out of the folders in that pst file, or just make a new MAPI profile. I’d probably recommend the latter. There are instructions to help you remove and create a new profile.
If you’ve got contacts, calendar entries, notes, or tasks, you can copy those from your backed up data file into your new one by using the Open Outlook Data File option from the File->Open menu, just navigate to the backup copy of the .pst file you saved.