Someone working for me converted some paid RHEL servers to Centos when the subscriptions ran out.
He pieced together instructions from a few places that let him change the whole update mechanism from RHEL to Centos without needing to remove or reinstall anything other than the update mechanism, so as new updates were available, the system gradually became Centos. Since updates and support are the only things missing from RHEL when the subscription expires, there was no need to remove and reinstall.
The whole transition was very smooth and I'm very happy with using Centos instead of RHEL. So I can give my feedback on the results of switching. I can't really give you meaningful feedback on the process of switching, because I wasn't the person doing it and the person who did do it did not find comprehensive instructions in one place that would tell someone at my skill level how to do it.