There is, though it's not implemented as a neat API function (yet), so if you're using the official release from when this was first posted you can't do it. It looks like this:

(update variable (where {:name "cats"}) (set-fields {:unit "encountered" :fn "non-negative?"}))

This will break preexisting data if the prior validator wasn't a strict subset of the new validator. Converting variables into a new representation sanely and easily is something I plan to add in.