Thanks all for your thoughts.
End of the day, SQ says, if I 'change' the ticket to J class, they'll NOT redeposit the difference in miles (57.5k>75k)!
My only option is to 'cancel' my current reservation and issue a new ticket from scratch.
Problem with that is, there's no more availability on the LH flight, and once I cancel my existing reservation I'm SOL.
What a stupid policy.
Can't understand them.