How do we prevent changes to Purchase Orders generated from approved Requisitions?

 You would think there would be a link from an OP generated from an OR, to prevent changes after the approval, but it appears not.  I looked at possibly using approval processing on OP's also or using Detail Line protection in P4310, but both of these 'solutions' would have to be 'all or nothing'.  What I mean there is if we turned on either of these functions, we would need to do so in all versions to effectively close the loop, and that's not something we prefer.

Any suggestions?