Symptoms
Downgrade order shows positive amount, even though the correct document (credit memo) has been created as a result of its processing. Additionally, invoice attached to the downgrade/upgrade order that includes downgrade items shows positive integer for the number of downgraded items.
Cause
That is expected behaviour for the downgrade orders.
Resolution
The only way to change the behaviour is to switch to Change orders instead of Upgrade/Downgrade (refer to kb #118187 for the details for the procedure).