Powershell replace text in multiple files lookup CSV

Hiya guys

After some guidance please.

I have a CSV with following details

MailboxName, PRFNAME,TempConstant

usera, a, usera.prf,TEMPLATEPRFUSER

userb, b, userb.prf,TEMPLATEPRFUSER

userc, c, userc.prf,TEMPLATEPRFUSER

Im after creating multiple copies of a prf file with the PRFNAME

Import-Csv $UsernamesCSV | % { Copy-Item "C:\TemplatePRF\Template.prf" "C:\TEST\$($_.NewPRFName)"}

This creates multiple prfs named correctly based on the prfnames provided in the CSV

Now I want to search for mailboxname=TEMPLATEPRFUSER and replace to "MailboxName" from my csv.

so it will be
usera.PRF
ContentsMailboxname=user, a

userb.prf
contents
mailboxname=user, b

So the common field to replace will be TEMPALTEPRF but replace with the relevatnt mailboxname depending on prf name.

After looking around I found the following

Param (
    #$List = "C:\TemplatePRF\mailbox.csv",
    $Files = "c:\Test\*.prf"
)
$ReplacementList = Import-Csv $UsernamesCSV;
Get-ChildItem $Files |
ForEach-Object {
    $Content = Get-Content -Path $_.FullName;
    foreach ($ReplacementItem in $ReplacementList)
    {
        $Content = $Content.Replace($ReplacementItem.TEMPConstant, $ReplacementItem.mailboxn)
			}
   Set-Content -Path $_.FullName -Value $Content
}

At the moment all the files will then have the content "mailboxname=" set as user,a and it does not seem to loop through the remaining and replace correctly.