Unfortunately this functionality was deprecated from K2 Five onwards.
K2 Management currently doesn't allow for bulk redirection. Please vote for the idea below, which will then also subscribe you to updates for this feature.
What version of K2 are you using?
It depends on the situation since redirect is not the same as delegate but maybe setting the old users Out of Office and putting in the new user as a delegate may be good enough.
Programatically it can be done as well.