I had two vCenter environments that needed consolidation and with the 4.1 main support ending it became even more urgent to upgrade the environment to 5.0. This upgrade was completed along with consolidating the two vCenter environments.
What I Learned
This was not my first upgrade but with the consolidation became a little different than the past upgrades I had performed. The jump from 4.1 to 5.0 is not a major one like upgrading to 5.1. or 5.5.
Lesson 1 - Interoperability & Planning
One of the main areas I have seen get missed during the planning stage is performing the interoperability matrix's with vendors and VMware in order to make sure that the hardware you are upgrading is supported. Vendors have created some great online tools to perform this task although it can be time consuming it will save you troubleshooting the environment after upgrade.
Lesson 2 - VMware Supported Sign Off
Once you have your plan in place open a case with VMware to review the steps with you. They will take the time to review the environment and your upgrade plan to insure that you did not miss any steps. I also found that having a VMware Engineer on the call while you upgrade saves time if you do run into issues.
Lesson 3 - Virtual Distributed Switches
I had to migrate the ESXi 4.1 hosts to our 5.0 vCenter environment as part of our consolidation and upgrade path. In order to accomplish this I had to migrate the vms on each host from a virtual distributed switch to a local virtual switch. This is easily accomplished with PowerCLI scripting. Scripting in my opinion is the only way to ensure that no mistakes are made.
Unfortunately when moving from 4.1 to 5.0 you cannot bring the virtual distributed switch over like when you move from 5.0 to 5.1 or 5.5 so you have to get creative.
Lesson 4 - Easy Upgrade
Once the vms on each host had migrated to a local virtual switch the next step, for consolidation, was to disconnect the hosts from the 4.1 vCenter, add the hosts to the vCenter 5.0 environment and place them in their own cluster.
At this point the upgrade becomes very straight forward. I created a new upgrade baseline using the ESXi 5.0 Update 3 iso and attached that to the host that required the upgrade. This is only one of many upgrade paths to 5.0.