How to enable an existing org as OPM enabled org with transactions existing in it

 We are unable to make an inventory organization as process manufacturing enabled after the transaction records exists in it. OPM Financials has not yet been configured and discrete costing is also not used currrently for the org. We had the understanding that we can make an inventory org as process manufacturing enabled at any time before the costing is done. Now when we are trying to make the org process manufacturing enabled,system doesnot allow us to do so.

Kindly guide us to any solution for making this existing org as OPM enabled org,as previous two years shipments have been made from this org in order management,and now we are starting OPM implementation.