valuation method for item has been changed after base document was added

Hello All,

Please i get a "valuation method for item has been changed after base document was added" error when i try to create an A/P invoice. This A/P invoice is supposed to be created based on a GRPO which was created previously. I edited the Valuation method on all items from standard to FIFO sometime last week, then changed it back again to "Standard" valuation method on that same day. Now, hundreds of GRPOs are pending due to this error. Please can i get a way around this error?

Thanks

{Note: I have attached screenshot of error for further clarity}