Hi BenAus
If set up correctly, when using 'I Inventory', what gets posted to the Cost of Sales account is from Sales. When you purchase, the full cost gets posted to the Inventory Asset account. Then when you sell the items, the cost of the sold items is moved from the Inventory Asset account to the Cost of Sales account.
If you do not use 'I Inventory' then your purchases get posted to the specified Expense account (some use Cost of Sales).
So I would suggest your prior year P&L was where Items were NOT set to 'I Inventory'.
If you can't get good assistance from MYOB, try someone like MYOX Services. Might be a small fee but may save you hours stuffing around. Esp if during the migration there were other things 'missed'. Get some experienced eyes to look over the file, particularly if you think the Cost of Sales figures are wrong.
Regards
Gavin