As you correctly state, the goodybags changed a few days ago.
If you bought your £20 goodybag before 11 July, it has 9GB of data then slows down between 08:00 and 23:59 daily.
If you bought your £20 goodybag on or after 11 July, it has 20GB fixed data allowance.
If you want the one with 20GB then slowed down data, it will cost you £25.00 but you cannot change the goodybag you've already got, so you wil lhave to wait until your next goodybag starts. Go here to select the one you want next https://giffgaff.com/profile/payment-details#goodybagsettings
Important: If you do not change the selected goodybag before 19:30 on the night your current goodybag expires, it will renew as the £20/20GB fixed data.