Unless seller has offered collection, it is not really possible to change, as seller then has no proof of delivery. For a proper collection buyer is given a code by ebay to pass to seller, and this proves delivery.
Changing from posting to collection gives seller no protection.
The long winded way of changing is for seller to cancel sale and relist with collection as means of delivery. This means being refunded and having to pay again, and refund may take time depending on how you paid.
