Hi Kelvin,
You may use a vSphere 6 design for your VCDX, even if the current version is still VCDX5. So you are good by updating it to the v6. However, note that you will still earn the VCDX5 if that is the version you defend under, although it's a simple upgrade to get to the new version (no re-defense required.)
Although I don't have a specific date for the VCDX6 release yet, it will be dependent on the prerequisite VCIX6 releases, which are targeted for Q1.
Thanks,
Karl