An attendee purchased a ticket for £10, and then decides some time later that they want to upgrade to a higher-priced ticket for £100. They are wondering - will they be charged £110 (£10 for the first ticket + £100 for the next) in total or £100 (just the price of the higher-priced ticket) in total?
Attendees can easily browse and purchase new ticket types without going through the entire registration process again. This can be done directly from any event page. As an Event Organiser, you can find and share a direct URL to the new ticket browsing sidebar. Just head to Revenue > Tickets to copy this URL and share it with your attendees through email, social media, etc.
Please note that they will be charged for both tickets as they are counted as separate transactions. They wouldn't automatically be refunded the first ticket price that they paid. It's something to consider when deciding on your ticket prices.
However, as the event organiser you could choose to refund their original ticket.
Alternatively, you could consider creating a coupon code to discount their second ticket. You have the option to set the coupon discount either by percentage or by a fixed amount.
Something to note: if an attendee uses an affiliate link to purchase the first ticket, and then proceeds to purchase the second ticket, both sales will be credited under the same affiliate link. This happens because once an attendee clicks on an affiliate link to get to HeySummit, the affiliate code will be automatically added to their cookies in the browser (60days), and will be valid no matter how many times the same attendee purchases a ticket.