What happens when a virtual machine whose VMware tools version has been updated to the latest version is moved to a host with an older version VMware tools version? This could happen if the virtual machine exists in a cluster with DRS.
It will not create any problems AFAIK. The version of the host only matters when you install the toold as it fetches the tools from ESXi. After that it will only work inside the VM, and should not create any issue when migrated on other hosts.
Most VMware Tools versions are compatible with older ESXi versions and vice versa. You can find supported combinations in the VMware Product Interoperability Matrix
Select "VMware ESX/ESXi" in (1), and "VMware Tools from VMware ESX/ESXi" in (2).
André