If you were downgraded from a paid business class fare, you should receive some sort of cash refund in addition to your voucher to use later.

If you were lost a space-available upgrade, then you should get your upgrade instrument back plus some sort of "downgrade kit" - in your case, the R/T voucher to use later is actually better than what many carriers give you.

Oh, and the guy who tried to rip lounge access from a passenger who had just been involuntarily downgraded should be fired. That's just low rent.