Your 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're 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?
The attendee will need to go through registration again to get the higher-level ticket. 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 organizer you could choose to refund their original ticket.
Alternatively, you could consider creating a coupon code to discount their second ticket. However, please note that our coupons calculate discounts in % terms, not specific amounts, e.g. $10 or £20 off, so this may not be simple.
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 would 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 browser, and will be valid no matter how many times the same attendee purchases a ticket.