I am taking your ticket is on SQ ticket stock. Only SQ can rebook you, the underlying ticket is not going away. If and when you rebook, the eVouchers are credited back to your Miles&More account. You can opt to collect miles in any StarAlliance program you want. You will get miles for the class booked and not for the class traveled. If you are flying on SQ metal in J upgrading to F you can even double-dip and get PPS value for Biz.