Copying a VM between Macs results in repeated installation of Parallels Tools

Hi, have a Mac at home and another at work. I regularly transfer a Windows 8.1 VM between them by opening the PVM package and copying the HDD file across using an external HDD. Worked fine under Parallels 8, but since upgrading to Parallels 9 I find that Parallel Tools wants to reinstall and reboot the VM every time I open the VM on the other Mac. Any ideas how I can avoid this? I'm using the same version of Parallels 9 on both machines, but one Mac is Mountain Lion and the other is now Mavericks.
Ta.