Reward upgrade S - G, online facility?

Evening All,
I have been trying to search online to the answer to this question but can't find anything on the VS website.
If I purchase a S fare online and G's are available, can I upgrade to a G during the booking process or would I need to speak to somone in the call centre?
Cheers
I have been trying to search online to the answer to this question but can't find anything on the VS website.
If I purchase a S fare online and G's are available, can I upgrade to a G during the booking process or would I need to speak to somone in the call centre?
Cheers