vSphere 6.0 has been around for about a year now, but VMware's largest customers are usually one or even two versions behind. With the recent release of Update 2 it looks like the 6.0 version has gained the stability and maturity that enterprise customers are waiting for. This is the reason why I just did extensive testing of vSphere 5.5 to 6.0 upgrades in the lab.
The main challenge of such an upgrade is to transform your vCenter Single Sign-On (SSO) setup into a topology that is fully supported and not deprecated. With vSphere 6.0 the SSO component is now part of the new Platform Services Controller (PSC) role that can be separated from the remaining vCenter services. In fact VMware recommends doing this whenever you want to have two or more vCenter servers in the same SSO domain which is a prerequisite for the new Enhanced Linked mode. The separation of SSO was already possible with vSphere 5.5 (although only with the Windows version of vCenter, but not the VCSA 5.5), but I think most people wanted to keep it simple and installed all vCenter services on the same server. So if they have multiple vCenter servers installed in this way then they need to switch to one or more external PSCs now.
There are many ways and orders in that you can - on the one hand - upgrade all components to 6.0 and - on the other hand - switch to an external PSC/SSO model. But only few of them are documented and supported by VMware. Their general recommendation is to transform into a supported topology first, and then do the upgrade of the PSCs and vCenter servers. KB2130433 e.g. describes how to upgrade/migrate two vCenter 5.5 servers with embedded SSO into the same SSO domain. This and other migration scenarios involve re-pointing your vCenter 5.5 server to a newly installed external SSO 5.5 instance.
So when preparing the upgrade of a complex vSphere 5.5 environment to 6.0 you will sooner or later stumble over KB2033620 which describes how to do this re-pointing. Unfortunately this KB article and the tools that it refers you to are very poorly written and full of issues. Some of them are mentioned in the KB article itself with workarounds to follow, but a lot are not ... Here is a list of the most annoying issues with KB2033620 and how to fix them.
