I would ad that for the other 400, you may have to restart some of the services they apply to in order to get the updated version running.
1. start with (ideally 2) full backups.
2. get spare PC and install the backup.
Ideally, you'd want the same basic HW (to check kernel+drivers) and just enough disk space to do basic app tests
3. you can change the RH subscription to point to the new PC.
Subscriptions are not locked to a machine, just the total number of machines registered at any one time
4. install all updates on new PC
# as root user
6. when all good, re-subscribe live prod, do backups again (it will have chnaged0 and update prod
7. you should find out how many subscriptions you have and ensure you have at least one for each prod and at least one for a single dev/test box.
Actually, you can cheat a bit by using Centos on some/all of the boxes.
Its a free rebuild of RHEL, includes updates, but no support.
As you can see, you could skip some of those steps, but its high risk.
If this is a serious server, then doing it properly is the cost of doing business.
I'm sure you have the equivalent steps for your MS servers.
Can't remember if I've pointed you at this before, so http://www.linuxtopia.org/online_boo...ion/index.html