When you create a Vendor Contract (and a Purchasing Scheduling Agreement), the system makes a copy of Material Master data.
If you change the Material Master, the Contract/PSA will not be changed. This is a problem, because all the Goods Receipts will be made based on the PSA information.
Traditional approach should be to delete PSA (after being sure that ther is no invoice, GR or Schedule Lines created), then delete the Contract, and create all new documents.
Here is a method to change in mass the conversion factor, direct in the contract/PSA.