Originally Posted by StarG
AFAIK that is possible only with miles, not by paying the difference between J and Y.
LH has started *A upgrade redemptions on SQ, so that may be the way to go for you.
StarG is 100% correct. Individual segments of a RTW are not upgradeable by paying cash. Either the whole ticket is upgraded - or not at all.
Edited : LH Miles are not be an option: only Y and B are upgradeable using LH miles. Being in Y - you'd have to be booked in M class, and have either voluntarily downgraded without surcharge from a CRWSTAR fare, or paid a Q surcharge of USD 100 from a YRWSTAR. Either way - you can't upgrade with LH miles.