I think you were specific enough...every suggestion so far should do exactly what youre wanting to do. Did you look at the UDF that Doc posted? In your case...you could use the UDF and do something like this...

Code:
$null = PathMod("c:\changeme2","c:\changeme3")

Function PathMod($CValue, $RValue)
	Dim $Array, $aCntr, $KeyVal2, $fCntr, $stat, $KeyLoc, $Keynam
	$aCntr 	= 0
	$fCntr 	= 0
	$stat  	= "FALSE"
	$KeyLoc 	= "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment"
	$keyNam 	= "Path"
	
	If $CValue = "" OR $RValue = ""
		Select
			Case	$CValue  = "" 
				$Pathmod = 2
			Case $RValue  = "" 
				$Pathmod = 3
		EndSelect
		Return
	EndIf
	
	$KeyVal 	= ReadValue ($KeyLoc, $KeyNam)
	$Array    = Split($KeyVal,";",-1)
	For Each $Element In $Array
		If $Element = $CValue
		  	$Array[$acntr] = $RValue
		  	$stat = "TRUE"
		EndIf
		$acntr = $acntr + 1
	Next
	For Each $Element In $Array
	  If $fCntr   = 0
	  	$KeyVal2 = $element
	  	$fcntr   = 1
	  Else
	  	$keyVal2 = $KeyVal2 +";"+ $Element
	  	$fcntr   = $fcntr + 1
	  EndIf
	Next
	If $Stat = "TRUE"
		$RC = WriteValue($Keyloc,$KeyNam,$KeyVal2,"REG_SZ")
		If $RC = 0
			$PathMod = 0
			Return
		Else
		   	$Pathmod = 1
		   	Return
		EndIf
	EndIf
EndFunction