Perform the following steps to configure the policy:
1- Create a new Workflow and go to Configure
2- Click on Select Operation
3- Keep Target object type as User and select Operation that starts the workflow as Modify Properties
4- Click Add and look for Last Name with LDAPDisplayName "sn" and click Finish then on next window click Save Changes
5- From the left pane drag and drop Update and then double click to open
6- Go to Target properties and click on Add property then More choices from the drop down
7- Look for Logon Name (pre-windows 2000) with LDAPDisplayName samAccountName then select it and then hit Ok
8- On the next screen let Action be Set then click Define and select Value generated by rule expression
9- Click Add and then click on Property of object from workflow data context
10- Set Target object to Workflow Target
11- Click on Target property and then in dropdown menu select More Choices
12- Look up for First Name with LDAPDisplayName of givenName, select it and then click Ok on next window select Ok again
13-Click Add entry and select Text String then in the field add a dot then click Ok
14-Repeat step 9, 10, 11, & 13
15-Look up for Last Name with LDAPDisplayName "sn"
16- Click Ok and on exit click Save on Workflow window
Try changing any user's last name for testing purpose and the 'samAccountName' should update as well.