I am going about this the other way. Please tell me EXACTLY what the SQL below will do. This is a check before I push the button and jump into disaster (hopefully not). That's Mr. Trebek to you.
INSERT INTO dbo_Mfg_Part_Master ( UDFKEY_49 )
FROM dbo_Mfg_Part_Master INNER JOIN qryExistingMatchingMPNs2 ON (dbo_Mfg_Part_Master.MPNNUM_49 = qryExistingMatchingMPNs2.MPNNUM_49) AND (dbo_Mfg_Part_Master.PRTNUM_49 = qryExistingMatchingMPNs2.PRTNUM_49);