There isn't a direct way to do this. You would need to use WF or the SDK to scan through the repository, identify all the rights currently assigned to the first user and replicate them to the second user. You could also theoretically do it manually by checking the db tables for all entries containing an ACL for user A and then manually adding user B. All access rights are configured based on the users SID, so they can't just be transferred.

For what it's worth, this is why I often recommend setting up as much security as possible through Laserfiche Groups, even if the environment is all windows authentication. Then the security is based on the role, not the user, and you would simply need to swap in the new user into the group when a situation like this happened. As such, I recommend using Groups as much as possible, even if it's a group of one. That may not have worked for your scenario, and I realize that doesn't help you in this specific situation, but it may be a way to avoid having to do this a second time in the future.