"Instead of making "Shada" a part of season 18, Nathan-Turner decided to make it a Christmas special. First, he had Adams rewrite the script, reducing its length to approximately 100 minutes (or a third less than its original length), as well as making it non-episodic. Next, Nathan-Turner needed one more filming session. Even with the rewrite, the production had to have some additional linkage filming. Finally, he had to bring back the principal actors.That same issue also has a look at the original script for Star Trek IV: The Voyage Home when it was a vehicle for Eddie Murphy.
"Although "Shada" was close to being finished, the planned special never came off. The actors were no longer under contract and not all of them could return due to other commitments.
"Besides the cast difficulties, Nathan-Turner wasn't allowed the additional recording session needed to finish the production, effectively writing finis to "Shada."
TV The Internet Archive recently uploaded back issues of what was one of the best sci-fi magazine around, Starlog, which ended publication in 2006. It's not complete, the latest issue is 224, but what's there arguably covers the golden period. It's in Starlog that I originally read about Shada, the Doctor Who story written by Douglas Adams, which was never completed and glancing through, sure enough, here's that very article, published as part of a special time travel issue: