I want to move a project (from my Hobby account) to a new team to transfer ownership to my client. But for that to happen I need to pay for Pro. Ideally I'd want to transfer the project to the new team, add my client as a member and have them handle billing.
As I recall I've done this before, but it doesn't seem to work anymore.
Hi, ! Welcome to the Vercel Community 
From the relevant documentation:
Projects can be transferred between plans as long as the project owner is a member of both the source and destinations of the transfer. For example, a project can be transferred from a personal account (Hobby) to an Enterprise account, and vice versa.
https://vercel.com/docs/projects/overview#transferring-a-project