Can I now only use the NFC-sticker to validate my ticket purchased in BudapestGO?

No, code scanning validation is still available. NFC is a new ticket validation method you can try. Be careful not to confuse it with the traditional code sticker, as you cannot validate your ticket with that in one go. To scan the code sticker, you'll still need to log into the app and scan the QR-code using the validation button on the side of the fare product.