Here is the answer that I got from another forum:
VirtualPC and Virtual Server used to have a VMAdditions (just like VMware's VMTools) - and like the VMware VMTools they did not play well on other hypervisors.
I bet that is the legacy advice. Hyper-V to VMware should involve nothing special as the Hyper-V Integration Services will not detect Hyper-V and thus they will not load. Not the way that things go when moving off VMware.
and then:
There should be none to concern yourself with. There is no user state component to the MSFT tools, they are all driver based. So if the drivers don't detect the 'devices' they don't load. XenServer vm tools work the same way. It is the VMware tools that insist on loading. And in the case of a VMware VM on another platform there are services and user mode monitors - they will all be broken. If it is the old VMAdditions - like the VMware VMTools - they can only be uninstalled when they are running on their platform (either VirtualPC or Virtual Server in this case) - not any other time. But it is the HAL that you need to watch out for. So force a HAL redetect in the VM.