If you leave the entry name in ReadProfileString() blank you will get all the entries in a section. If you leave the section name blank you will get all the sections in a file.

Use this technique to enumerate all the sections and entries.

Here is a simple (untested!) example:
Code:
$sMyIni=".\serverlist.ini"
$sOldString="Server1"
$sNewString="Server2"
 
For Each $sSection in Split(ReadProfileString($sMyIni,"",""),Chr(10))
   If $sSection
      "Checking section "+$sSection+@CRLF
      For Each $sEntry in Split(ReadProfileString($sMyIni,$sSection,""),Chr(10))
         If $sEntry
            $sValue=ReadProfileString($sMyIni,$sSection,$sEntry)
            "Entry "+$sEntry+" has value "+$sValue+" status:"
            If $sValue=$sOldString
               " **UPDATED**"+@CRLF
               $=WriteProfileString($sMyIni,$sSection,$sEntry,$sNewString)
            Else
               " No change"+@CRLF
            EndIf
         EndIf
      Next
   EndIf
Next