I imagine it was whoever wrote the script.
As a rule, that's not a safe assumption. In TV, writers are in control, but in Hollywood feature films, it's usually the directors and producers who make the creative decisions and scriptwriters are seen as little more than hired contractors brought in to put those decisions on paper.
Of course, it's a different matter if the scriptwriter
is the director or producer, and that's the case here; TUC's director Nicholas Meyer is also one of the credited screenwriters. And we know from his collaborator Denny Martin Flinn that it was Meyer's idea to bring in Dorn:
http://memory-alpha.org/wiki/Worf_(Colonel)
"The genesis was really Nick [Meyer] saying, 'How about if we get Michael Dorn to play the part of Worf? and everybody said 'Nick, The Next Generation is 75 years later!' and Nick said, 'Okay, we'll make it his grandfather' and that was it. Nick had not created the part of Worf for a particular actor, but we got Michael Dorn and they explained to him he couldn't play himself." (Captains' Logs: The Unauthorized Complete Trek Voyages)