Changing a changed event amount is not recorded properly

The partial payment code that was added for events has broken the way that changes to already changed event payments works. As of 4.7.9, if one has an original ticket price of $1000, then change it to $500, then change that to $1500, the resulting total amount is $-500, not $1500.

select 1000

• 1000
  Select 500
• -1000
• 500
  select 1500
• -500