If your upgrading the ESX from 4.0 to 4.1, you should have Update Manager version 4.1 and Update Manager 4.1 is compatible (VMware Product Interoperability Matrixes ) to work only with 4.1 vCenter server.
Also ESX4.1 is not compatible to work with vCenter server 4.0
So recommended upgrade would be
- Upgrade vCenter server to 4.1
- Upgrade Update Manager to 4.1
- Upgrade vSphere infrastructure client to 4.1
- Upgrade ESX to 4.1
Upgrade to latest Update 4.1Update03.
-Nithin