BANANAS is right you really do need to get the paxing formalised into a written agreement.
Sadly MEL has always been a problem, in my day due largely to one person in Traffic who could not disguise his dislike of crew, he may still be there!
Suggest you keep an accurate record of who gets upgraded and when, the IFS gets a copy of a message that shows all this, including Silver Kris members who have been upgraded but the ones you need to know about are the ones labelled 'Involuntary upgrade' - that means they have no entitlement and have been upgraded to create space in economy, this should only happen after entitled Staff have been upgraded, you really need to check with the IFS each time and keep records.