Here you go sir, 75% of your GUC. Enjoy!
Compensation via EC261 doesn't seem likely to lead to much, since there was no cost associated with the upgrade.
No, EC261/2004 downgrade compensation of 75% applies to the cash part of the ticket as well.
If the ticket (segment) costs EUR 500 in Y + GUC -> DL has to refund EUR 375 + 75% of the GUC.
Ticket value is calculated according pro-rata great circle distance.