There seems to be a major difference in the case of pre-paid upgrades: Whereas before, to get into First one would need to PAY for I or C class and then use the voucher to get an A class ticket issued, now it seems that the original ticket needs to be issued in I or C class AND then to apply the upgrade to get A class. As I understand it, if I/C are full then you will not be able to get into First via the pre-paid upgrade route, even if there is availability in A.