There's definitely a way to do what you ask in the latest version of WordPress, but I'll warn you that like many things in WP it's rather confusing to figure out where you can make this change and how to actually do it.

Before I start, though, I'll add that I too have two accounts set up for my WordPress blog -- GoFatherhood.com -- so that when I'm logged in on a daily basis, adding comments, writing posts, etc, I'm not actually the administrator of the blog. Saves me from making dumb mistakes that could be painful to fix. When I need to administer, I just sign out, sign in with the admin account credentials, and do what I need. Ideally, I then remember to log out and log in again as a regular user, but not always. Like you, I sometimes end up with a blog post by "admin". Not good.

To fix it, ironically, you'll need to be logged in as the administrator of the WordPress blog.

Do that, then choose "Posts" --> "All Posts" and find the entry you want to tweak. It'll look like this:

You'd think that if you clicked on the article author name (in this case "taylor") it would let you change it, but it doesn't. Choosing "Edit" or "Quick Edit" won't let you change it either.

The trick is to check the box adjacent to that particular entry, then from the "Bulk Actions" pull-down menu, choose "Edit":

The result offers a lot of things you can tweak that you probably didn't realize you had access to on a post-by-post basis:

The field we want to change is Author, but notice you can change categories, add tags, disable comments, change the post status, and much more. Darn helpful, really!

Click on the "-- No Change --" menu adjacent to Author and it'll show you the options. Pick the one you want...

Now click on "Update" and you've just changed the author of your WordPress blog post. Congrats!

Hope that helps you out. Good luck with WordPress. If you need it, I do have a lot of WordPress and Blogging Help here on AskDaveTaylor too...