Chase Sapphire Reserve bonus after 48 months

My 4 year anniversary with the CSR is coming up. I couldn't re-earn the sign-up bonus during that time, but now that's going to be gone how do I earn it again?

I've read that I could down grade and upgrade, but is there a step by step detail so I don't mess it up? I'm not finding much about that except for thus: https://upgradedpoints.com/credit-cards/reviews/chase-sapphire-preferred-card/upgrade-to-chase-sapphire-reserve/