YUM/DNF can do the whole thing without much fuss.
That question is impossible to answer with 100% certainty. There are no 100% guarantees. It all comes down to how much you trust the Rocky Linux crew and also how much you trust the project’s ability to raise the necessary funds.
There is also the option to buy licenses for Red Hat Linux.
I'm not sure that's a fair question to a new project. If you asked that recently about Windows 10, you'd have a different answer than today.
But seriously, Rocky was started by the person doing CentOS before and unhappy with changes. They're at the very least not planning to go away.
Windows has, before Windows 10, had a support cycle of 5 years for feature updates and another 5 for maintenance. XP was supported past that term for (too) long because people refused to or were unable to switch, and 7 has befallen a similar faith.
The news media recently found the support cycle page again after the leaks about the upcoming Windows 11 release, but none of this should be a surprise for people who need to look into long term support for Windows.
The difference here, or course, is that a big company like Microsoft is more likely to stick around than a small, open source project. They currently don't have any plans to stop support, but who knows what the future may bring. Covid is a stark reminder of how the world can change in just a few months.
The AlmaLinux team (another RHEL8 clone created by CloudLinux in the wake of the CentOS news, already released/available) has created a tool to do the conversion. Rocky should have no problem, if Alma can do it Rocky can do it.
A. How easy will it be for the unlucky people that have installed Centos 8 to convert it to rocky Linux?
B. Will rocky Linux fulfill its promises? I mean if I install rocky Linux 8.4 will I still have a supported OS at 2029?