I'm sure this is asked a lot, but I can't find it via the search.

Is it possible to change the author of postings? If anyone can point me in the right direction, I'd appreciate it.

Yes you can, but one entry at a time.

Go into the MT control panel, click on Edit Entries, then at the bottom of the intro paragraph is a hyperlink "Open power editing mode." Click that, and you'll see a table of your entries and a column for Author that has a drop-down selector to change the author, and a Save button at the bottom.