Yes at check in for an LH/LX/OS/LO (ignoring the small LH Group carriers here) flight using M&M miles. Come on, Star Alliance upgrade awards (what the OP is trying to do) need to be done before the flight. You can do it via LH web site (need to provide SQ eticket, booking class etc.) and if there is availability it will tell you before taking your LH miles or eVouchers. Of course calling LH will be as good. I have done it on the morning of departure for SQ325 (the flight ) from B booking class to Biz using 2 LH eVouchers, but M&M miles would have worked too.

How will SQ check in staff process an upgrade for a customer flying SQ using LH M&M miles? Cannot lah.