I think it may be simpler than expected. Just update you existing $GivenName
like this:
$GivenName|%{$_.givenname = (Get-Culture).TextInfo.ToTitleCase($_.GivenName)}
Then you can do a $GivenName|Export-CSV .\Documents\UsersF.txt -NoTypeInformation
and there's no need to create a whole other variable for the exact same info.