From the list of the Portal user roles, you can change a role to the users (contacts) that were already assigned and can replace with another role as you want to assign.
By clicking on the Replace Role option, a pop-up will appear to select another role from the drop-down list.
Once you select another role, it will replace the current one and assign the new role to all the contacts (portal users).
Note: Here in the drop-down list, only those Roles will appear whose Relation Type is common.