I had to go through this once before. I was on RedHat 7.1, and that got dropped. So went to RH9 which was the very latest available at the time. Now it is time again.
To be honest the only safe way of doing things is to have two servers. Upgrade one to newest OS, move data to that. Then when the time comes again, do it the other way.
Alternatively just backup to somewhere safe, upgrade OS (from scratch), then restorew data.
I'm sure it is possible to do a "live" update, but it is bound to screw something up, and I'm sure Plesk won't be happy as you'll have a version for one OS installed when in fact you now have a different one if you upgrade "live".
Maybe someone has actually done it? I'd be interested to know too out of curiosity.
Faris.