Lost In Time was tackled far earlier on than they expected - due to the return of Day of Armageddon. Had it not been for that, who knows how the episodes would have been handled. Again the return if Underwater Menace 2 forces the issue, how to handle half the story. We know 2 episodes a time aren't too many to do, we know underwater menace and moon base have had commentaries recorded. So if both of those are rereleased, that will be 3 of the Lost In Time 18.
Wouldnt be surprised by further returns in the next three years either.
By the way, the death count in season three is staggering.