If you described and named the second, downgrading transaction more in terms of it being a return of the original upgrade (instead of giving it the separate name of “downgrade”), you could perhaps call it an upgrade “return allowance (upgrade cost less fixed restocking fee/depreciation).
(example usage from ‘Wiley GAAP Policies and Procedures’ by Steven M. Bragg, via ‘Google Books’)
For a single word that you could use with “downgrade” that implies that some, but not all of the upgrade cost is being returned, there’s “downgrade offset," which ‘Merriam-Webster’ defines as:
6 : something that serves to counterbalance or to compensate for something else; especially : either of two balancing ledger items