As everyone knows, old commitments count less than new commitments. The logic behind this is presumably that old committers may have forgotten about the site. The solution that people have found is to try to convince people to uncommit and then commit. This is obviously a hack, and not the right way for it to work.
Here's what I propose. When you are already committed to a site the button that usually says "commit" instead says "update commitment." Simply clicking it once would have the same affect as committing and uncommiting.
(I could even see the argument for having pushing "update commit" actually count for more than what you started out as. What better way to see if people are committed to visiting a site every day then having them click that button every day...)