Upgrade configurations allow for specifying a BalanceChange to be applied to an account as part of a network state upgrade.

Currently, the BalanceChange value is interpreted as a uint256 and always added to the account balance, but there is no validation that the configured value provided was not negative.

https://github.com/ava-labs/subnet-evm/blob/master/stateupgrade/state_upgrade.go#L34

Whether or not balance deductions are supported, proper overflow & underflow checks should also be added.