I recently had to update a number of drupal installations, some of them quite old. Here is a bash "few-liner" to migrate an install to CVS, making it much easier to update in the future.
cd your_drupal_dir
# replace DRUPAL-6-12 with the appropriate version tag
cvs -z6 -d :pserver:anonymous:anonymous@cvs.drupal.org:/cvs/drupal checkout -r DRUPAL-6-12 new_drupal
rm -rf new_drupal/sites
mkdir old_drupal
for i in `ls drupal`; do
mv $i old_drupal
mv drupal/$i .
done
# if you have drush
drush updatedb
# make sure everything's working, then
rm -rf old_drupal
Standard disclaimers apply, hope this helps some people!