So the general instructions are:
1. Call the helpline (they will charge you by the minute). Can't remember offhand what the number is, but it's on the website.
2. Tell them you want to upgrade. If they say no, ask to speak to a supervisor.
3. Supervisor should tell you how and should confirm the process.
4. You will then have to go to the VFS website and buy the priority service for $700.
5. You will have to email this receipt to the address given by the supervisor.
Other users have other tips and did other supplementary things, and others gambled a bit more, but this is the general process!