Assuming you wanted to build the e-mail address from the AD info, you could build that via First and Last name, then you’d do the following:

Assuming the e-mail addresses have already been built, and you just want to make the SIP URI match, it would be simpler: Just copy the e-mail attribute contents to the SIP URI.

This should get you started, though it isn't a complete solution for your question. It's from a script that modifies the SIP URI, but by changing the domain, not the user name:

Prereqs  - Need the OCS Admin tools installed on the machine (or run this from the front end server).

$SIPDOMAINTOMODIFY = “domain1.com”
$NEWPODSIPDOMAIN =  “domain2.com”

function ModifyUsersSipUri
{             
                Write-Host -foregroundcolor Green "Reassigning user accounts...";
               
                $users = gwmi -class MSFT_SIPESUserSetting;
                foreach($user in $users)
                {
                                $sipUri = $user.PrimaryURI;
                               
                                $newSipUri = $sipUri.Replace($SIPDOMAINTOMODIFY, $NEWPODSIPDOMAIN);
                                $user.PrimaryURI = $newSipUri;
               
                                Write-Host "Changing SIP Uri for:" $user.DisplayName "($sipUri --> $newSipUri)";
                               
                                $output = $user.Put();
                }
               
                Write-Host "";
}