Powershell question about Get-ADUser

Hello,

I have a csv file that has employeeID with first_name and Last_name.

employeeID   FirstName      LastName
-----------------   -----------------    ---------------
111                    John               Doe
222                    Jane                Jones

I also set an attribute in AD called extensionAttribute5.  If the value of extensionAttribute5=1 then make the name change else do not.

I have this script:
Import-Csv update.csv | ForEach `
       $employeeID = $_.employeeID
       $SN=$_.LastName
{
      Get-ADUser -LDAPFilter "(employeeID=$employeeID)"| Set-ADUser -replace @{sn=$SN}
}

But the problem is it will change for all users in the file.

How can I put a condition so it would be something like this:
Get-ADUser -LDAPFilter "(employeenumber=$employeenumber)"| If (extensionAttribute5=1) {Set-ADUser -replace @{sn=$SN}}

Thanks.