You can do an upgrade in a safe way by restoring 6.5 backup on the fresh install of Plesk 7. In-place upgrade is being tested right now - please note that we have to test every 6.5 minor version, plus combinations of initial version and subsequent 6.5 patches.
We just want to be cautious with potentially destructive in-place upgrade. 6.5.2 -> 7.0 is probably the easiest upgrade, but I would still recommend you to wait until we finish testing and release the final upgrade utility to the public.