The official story goes, that Gene Roddenberry made a frantic December 13th revision, where the character of Khan was first introduced:
https://www.startrek.com/en-un/news/guest-blog-the-evolution-of-space-seed-part-3
“Who can rescue this script and episode, now just 8 days from filming?”
“Gene Roddenberry, of course. It will be Roddenberry who fixes the final version of “Space Seed” without credit. Handwritten notes show that Roddenberry had some interesting ideas he wanted to add to the script that unfortunately were not included in the final result. One that is especially intriguing is the idea that Khan would have a problem with Spock because interspecies mating “might sully the human race.” This would have been an especially fascinating theme in “Space Seed” if it had been used, especially considering it is Spock who ultimately defeats Khan’s plans in Nicholas Meyer’s Star Trek II: The Wrath of Khan.”
I recently acquired a revised final draft from December 12th with all blue pages, and it already has Khan. The only mention of Ericssen, the Nordic superman, is on the cast list that reminds to change all mentions of him to Khan. The 12/12 script has only mentions of Khan (sometimes misspelled Kahn). This draft is credited to the original author Wilber and Coon. The 12/13 revision changes this up to Roddenberry and Coon.
The 12/12 draft seems to have all the pieces in place regarding Khan before Roddenberry put his name on the script. Without seeing the 12/13 script, it’s hard to say what he fixed at the last moment.
https://www.startrek.com/en-un/news/guest-blog-the-evolution-of-space-seed-part-3
“Who can rescue this script and episode, now just 8 days from filming?”
“Gene Roddenberry, of course. It will be Roddenberry who fixes the final version of “Space Seed” without credit. Handwritten notes show that Roddenberry had some interesting ideas he wanted to add to the script that unfortunately were not included in the final result. One that is especially intriguing is the idea that Khan would have a problem with Spock because interspecies mating “might sully the human race.” This would have been an especially fascinating theme in “Space Seed” if it had been used, especially considering it is Spock who ultimately defeats Khan’s plans in Nicholas Meyer’s Star Trek II: The Wrath of Khan.”
I recently acquired a revised final draft from December 12th with all blue pages, and it already has Khan. The only mention of Ericssen, the Nordic superman, is on the cast list that reminds to change all mentions of him to Khan. The 12/12 script has only mentions of Khan (sometimes misspelled Kahn). This draft is credited to the original author Wilber and Coon. The 12/13 revision changes this up to Roddenberry and Coon.
The 12/12 draft seems to have all the pieces in place regarding Khan before Roddenberry put his name on the script. Without seeing the 12/13 script, it’s hard to say what he fixed at the last moment.