The inventory account will be the reduction of inventory, it means financially you have been overstating your holding for 7 years, I doubt this I would guess these have been corrected off, so you could contra the COGS and inventory, the issue is you need to take each transaction individually and assess it. For example if the customer went bankrupt the liability should have been written off, it was not because it was never invoiced, meaning has it been declared as GSNI for all these years. As an accountant you would need to know what action was taken when financially, what financial impact the change you are making will have and then how you approach the closing off of each order. Although if no one has noticed this for 7 years you could probably just invoice it all off, not make a COGS adjustment and then write a load into suspense and no one would notice [:D]
Get this data into test, assess where the current financial transactions are, or where they were adjusted. Close the transactions, make your adjustments and ensure you are happy with how the net results is presented in the accounts. A massive job I would guess.