First thing to do is go to the "Manage > Entries" screen. Select the entry or entries that you want to change the author of:
From the drop down menu, select 'Batch Edit Entries':
Hit the "Go" button, and you will get to see a list of the entries you selected:
Now for each entry you want to change the author of, click the name in the 'Author' column and pick the new author:
Note: yes, you have to do this for each entry separately.
Note 2: don't forget to republish your blog to make the changes visible in the published part of your blog.
Note 3: only users that actually have the proper permissions to publish an entry on the blog will show up in the list.
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You could just use the much easier method of the Ghostwriter plugin - http://plugins.movabletype.org/ghostwriter/
I've used it on MT4 for quite some time and it's wonderful.
Beau's Ghostwriter plugin works well for this too - and it's a few less steps (though it's only for one entry at a time). It does have it's uses, though!
Thanks so much! Tedious, but straightforward :-)