Can one applied payment/receipt be moved to another AR Item on the same Invoice?

Yes, a payment applied to one AR Invoice Item can be moved to another unpaid AR Invoice Item by using AR Correction adjustment from the General Ledger tab of the Cash Receipt Item record. 

The CR item can only be moved over the new unpaid AR Item that is equal or greater than the original paid AR Item.  This payment cannot be split over two different AR Items on the Invoice.