If I get a Legendary Key from G2A after Oct 28. Will it auto convert to Special Edition? Or was that only a deal for people who had it before Oct 28th?

You needed to have purchased either the Legendary Edition or the Skyrim plus the all the DLC before the 28th in order to receive credit for the SE. If you had the necessary items, it will already be in your Library.

If you missed the windows, then you'll need to purchase it at full retail price.