VMware made a long-anticipated move July 22 when it announced that its ESXi hypervisor would be free. That doesn’t mean VMware customers can avoid pricey fees for support and management tools, though.

I don't know if this is limited to paid keys, but for my Enterprise keys I can convert from 6.0 down to 5.5 or 5.0 and back up again if needed. I do have active support, so that's possibly part of it.