No, you can't upgrade. This is from my email.
As a Silver Season Ticket Member you are unable to change to a 19-match Season Ticket level, such as Gold or Platinum.
You are however provided with a priority window to purchase tickets for the matches not included in your package (Arsenal, Chelsea, Liverpool, Manchester United and Tottenham Hotspur). You also have access to a range of benefits including access to purchase away tickets (subject to ticketpoints) and our Ticket Exchange and Ticket Transfer platforms.