I updated the copyright on the OpenMx website, and the source code. The source code update was done with a few lines of sed at the Unix terminal.
find . -name "*.h" -exec sed -i "s/Copyright 2007-2012 The OpenMx/Copyright 2007-2013 The OpenMx/g" '{}' \;
This finds files in the current directory (.) with the name ending in ".h" and executes sed inplace (-i) editing the copyright (replace "Copyright 2007-2012 The OpenMx" with "Copyright 2007-2013 The OpenMx"). Analogous commands where done in models/passing, demo/, man/, and src/.
"svn up" your source code copies!
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I updated the copyright at the bottom of the Drupal pages, too.
For future reference, the change is made through Drupal, via Site Configuration -> Site information, by changing the "Footer message" field on that page.
If we want to enable to PHP Filter module, we could make this auto-update using Drupal blocks, instead, but I haven't done that because I'm not sure what the consequences (for, e.g. security, stability, etc) might be. But the option is there.
This is how the footer update is done now. It's in Admin > Structure > Blocks > Footer > Configure.