A PowerShell script that will mass assign UPNs for users in previous domain.

mass-update-office-365-upns.ps1

$oldSuffix = "old-domain.com"
$newSuffix = "new-domain.com"

Get-MsolUser | 
Where-Object { $_.UserPrincipalName.EndsWith($oldSuffix) } | 
ForEach-Object { 
    Set-MsolUserPrincipalName -ObjectId $_.ObjectID -NewUserPrincipalName ($_.UserPrincipalName.Replace($oldSuffix, $newSuffix))
}