I have not officially upgraded the 4.1 licenses to 5 but when I just looked at doing that it sounded like I couldn't keep my 4.1 hosts. I am going to upgrade but some maybe not for a couple months.
What is the scoop on this?
There is nothing that will invalidate the keys you already have defined in vcenter for vsphere 4 hosts. When you upgrade your lics online, you will be granted new keys that you install that you will apply against esxi5 hosts are you bring them into your environment.
Also, after upgrade if you need to downgrage you can always do that.
You are supposed to stop using the old licenses after upgrading (I believe a warning will present itself to this nature).
That being said, during a transition phase I don't see how anyone would care outside of an auditor, and I'm not aware of any sort of call home feature being tied into any of the VMware licensing processes.
I went through this very same process a few quarters back, and nothing stopped working after we upgraded and consolidated our licensing. This allowed a nice POC environment to be stood up to ensure the logical design was correct, to weed out any upgrade issues, and get comfortable for the real upgrade to production.
1. Upgrade all your licenses and acquire the keys
2. Upgrade your VCenter
3. First thing after it: Cut/Paste the the VCenter 5 and ESXi 5 Licenses you've aquired
4. Upgrade your ESXi hosts to 5 whenever you want
5. When everything is done you may want to remove the 4 licenses