When I choose a Wordpress theme for a new blog, am I committed to it for life, or can I change it later without messing up links and archives? Thanks.
You can change the theme but any content that is coded into it will not appear on the new blog. Metadata and footer links are usually done that way but everything else is read from the database.
No you are not, but you may have to reconfigure things. My best advice is to download a couple themes you like. Then install them in the Themes Directory. Try adding some sample content and switch back and forth between themes. I recently converted one of Brian Gardners Themes to something else. I lost the Featured Content Plugin Gallery and had to reassign a few categories. It took a little time but it's not really that difficult. That is what makes Wordpress so Powerful the data is stored in the MySQL database and the theme methodology allows you to change presentation without destroying the data.