Can you upgrade ESX to ESXi... For example ESX 4.1U3 to ESXi 5.5 ?
Assuming the hardware is compatible of course.
I've always gone from ESX to ESX, like we started out with 4.0. Going to 4.1 was a breeze. Going to U1, then U2, then U3 was a breeze too. But now all the new vmware hosts are ESXi.