Upgrading (actually it installs a new kernel) via the RHEL tool 'yum' should be fine.
iirc, it keeps up to 3 kernels listed as available in your grub.conf.
There's a setting somewhere where you can amend that limit.
Don't think I'd try building your own from scratch, as RH heavily modify the 'stock' kernel before distributing it.
You should read the Release notes for each kernel if you REALLY need a specific feature/bugfix.