It's never a must to upgrade. If 9 runs fine for you and if your project is really big where you have to migrate everything maybe it's not needed to go to 10. But if cypress 11/12/13 comes out and you are still in 9 there will be a chance that you have to rework more at a later stage. Because Cypress will add more features im the future we already have cross domain testing(cy.origin) and session. I would just create a seperate branch in your project, upgrade to 10 and if it works without issues you can go from 9 to 10. If it will break things i will schedule some time with the team to discuss all new features in Cypress and when you want to migrate.