I have a 2nd class libero all zones abo. If I buy a class upgrade, can I pay 1/2 (as with a GA) or do I need full price?

Class I grade with libero abo

I have a 2nd class libero all zones abo. If I buy a class upgrade, can I pay ½ (as with a GA) or do I need full price?

No, you would have to purchase a full fare.
A reduced fare ticket would only be possible if you had the Half Fare Card in addition.

Kind regards,
Roland