Can I reassign a user’s account?

For data integrity purposes - it is recommended to delete the old user and add the new user directly into MMC (not merely swap the emails of the old user). Any Company, People or Deal records that need to be moved into the new rep's ownership can be done once the new user has been added. 

The deletion of the old user then addition of the new user ensures that Activities logged by the prior user are not credited to someone who did not complete them. 

Additionally - this adding of the new user will ensure this new team member receives the Password Set Up email to get started with their MMC account