Certifications are for major versions only, so there is a VCP6-DCV but I doubt there will be a VCP6.5-DCV.
There might be an exam based on vSphere 6.5, but it will still only earn the VCP6-DCV - so if you have that certification already there will be no reason to take a vSphere 6.5 exam (unless you need to recertify after 2 years perhaps).
This follows the same pattern as for vSphere 5 - there was a 5.0 exam (which also applied to 5.1), then a 5.5 exam - but only a single VCP5-DCV certification.
This is my situation currently; I have the VCP6-DCV, but it expires in October.
If it doesn't change to VCP6.5-DCV is it just a matter of it being a different exam number?
How does that work?
The vSphere 6.5 based exams for VCP6-DCV are due for release this month, and you will be able to pass the VCP one to renew your certification.
Yes, the vSphere 6.5 exams will have new exam codes.
Or you pass a VCP exam for a different technology area, or you pass a VCAP exam.