Global Data Plan extensions are only available up to the amount of data purchased in the latest transaction. 

i.e. If a user with 5GB in global data balance purchases a 3GB Global Data Plan, the user will have 3GB from the latest purchase and 3GB extended from the 5GB in its data balance. 

Please note that the extension policy only applies to Global Data Plans. There are no validity extensions for the purchase of Local and Regional plans.