Issue in script

msabat 

Which command do you use for migrating the users to Teams? As of CU8 for SfB 2015 and SfB 2019, you can use the following command: Move-CsUser -Identity "someone@domain.com" -Target "sipfed.online.lync.com" -MoveToTeams -Credential $cred

That command doesn't need a connection to msolservice or Skype Online. Therefor you also won't run into the timing issue.

If you still need to use the Skype Online module, you could use something like this:

$StartDate = Get-Date
foreach($user in $users){
    $CurrentDate = Get-Date
    If ($CurrentDate -ge $StartDate.AddMinutes(45)){
        $sfbSession = New-CsOnlineSession -Credential $cred
        Import-PSSession $sfbSession -AllowClobber -ErrorAction SilentlyContinue -WarningAction SilentlyContinue
        Enable-CsOnlineSessionForReconnection
        $StartDate = Get-Date
    }
Grant-CsTeamsUpgradePolicy -identity $user.UPN -policyname UpgradeToTeams -confirm:$false
}

To be on the safe side, it will reconnect 45 minutes after the first connection.