Spent some time chatting with Google support staff today and they sent me an email explaining the situation why some paid Google Drive subscribers have not been upgraded to Google One.

Email: https://i.imgur.com/DvYTVUz.jpg

TL;DR: If you have received promotional storage from things like security checkups, local guide rewards, chromebook promotional storage etc., they are still working on a way for users to keep this promotional storage and be upgraded to Google One. There is no timetable on when this will happen, all that I was told was soonTM

P.S. I tried the trick of allowing your current subscription to expire and then sign up again, but Google One was still not available for me. Guess we'll just have to wait it out.