In the past (1½ year ago) I had no issue getting invol. rebooked to flights with only the much more expensive standard award availability (promotional award availability had been depleted long time ago) but it may be that TK has changed/tightened the invol. rebooking rules for TK M&S award.

I doubt TK will rebook award tickets into revenue class but if the flight have standard/guaranteed award availability, rebooking should be possible.