I will suggest go for "Batch Input Recording"
It will be much easy to use for ME22N transaction.
You have wrote "I only need the LSMW to change the storgare location info on selective line items of the POs."
Do you mean to say that you have a PO with 5 line item,
10,20,30,40,50
but you want to change sloc for 2 line item only.
20,50.
Is it correct??