Don't worry, you do not need to upgrade your repository's plan in order to transfer it to another user. The Prismic support staff can help you transfer the repository.
First, the other person needs to setup a Prismic account. They can do so here: https://prismic.io/signup
Once they've done this, head to the prismic.io website and open up a chat with the Support team. Just look for the little chat button in the bottom-right corner of the screen.
You will need to provide the following:
- The email address for the account you wish to transfer ownership to
- The url of the repository you wish to transfer
With these things, the Support team will be able to take care of the rest!
Once the repository has been transferred, the other person will be able to upgrade the repo as they need.