It's so easy that you cannot mess up anything. Just make sure you did backup everything you could need from the old OS.
By the way, why did you need to wait the end of your phone contract to install GrapheneOS? Nothing prevent you to so before and (at least in Europe) the warranty cannot we voided for software change.
While Eluc is right PsySov, there is indeed no time like the present. If you have any questions, need any support etc feel free to reach out, happy to help.
Welcome aboard.
I was unaware. I was under the impression that contract phones were pretty locked down, but it's probably just from switching to a different providers SIM.
Either way, better late than never.
