Was that kind of fast?
That was 2014; if this was the original plan, they took their sweet time with it.
Red Hat also offered a conversion procedure the last time I looked, but it replaced every installed RPM with equivalents from its own repositories. This is quite violent, and it would be far from my first choice for a critical system.
The whole point of CentOS was to be 100% compatible with RHEL, right down to the packages and binaries. For literally this kind of scenario. I don't see how that's "violent." I'm also not sure how you can expect to convert from one distro to the other without replacing all (or at least most) of the packages.
The thing is, they were almost exactly the same distro, so in any meaningful sense you could switch from one to the other by switching the release files, some yum support stuff, I think the kernel?, and some miscellaneous branding if you cared. But 99% of packages were functionally the same, so yeah I don't see replacement being a problem per se, but it also didn't seem very necessary.
The code is the same, yes. The "binaries" are not, necessarily. The build environment isn't exactly the same and there have been bugs experienced in CentOS that weren't in RHEL and vice versa.
So it was never 100% compatible, more like 99.5%. The exceptions are rare edge cases but they do exist.