They cannot change the EMU part of the existing EU Treaty (via a so called IGC - Inter-Governmental Conference) w/o giving in to Cameron's demands for "repatriation" of certain competencies from EU back to national (or just UK) level!
1) If they offer Cameron what he wants, then other EU members, eg one of the other 9 outside the Euro, but not anti-EU, may raise a veto, ie veto the whole IGC agenda.
2) If they plan a Treaty that works outside the EU Treaties (in a format similar to the original Schengen way/model), then the contents of such Core EU/Euro Treaty can it seems to me can only add, not amend, articles of the existing EU Treaty.
Hence there is a Catch22 of sorts.
So what on Earth are (allegedly) Merkel and Sarkozy talking about?
In an IGC, be it EU or Euro one (see above), all will have to agree, not only Merkozy & "German thinking" others!
Bottom line: Euro membership was/is irreversible, those who do not get that are the ones in denial of legal, technical and other factors!
In speech in Strasbourg, Sarkozy called such plans a mere "intellectual exercise". IMHO they are not a trick. They are just that. Nothing feasible about them.