I read through a few times to make out how the migrates works. Before we start with the example, let's understand some of the acronyms for the track.
VCP-DCV = Data Center Virtualization
VCP-DTA = Desktop & Mobility (formerly VCP-DT = Desktop)
VCP-CMA = Cloud Management & Automation (formerly VCP-Cloud = Cloud)
VCP-NV = Network Virtualization
For existing VCP5-DCV, you can upgrade this without a class via a VCP6-DCV Delta exam (this is yet to release) similar to VCP5.5-DCV Delta exam.
For new users trying to get certified, you will be taking the VCP6-DCV inclusive of a class as usual.
Since there are already new some version 6 exams: VCP6-DT and VCP6-Cloud, already released (Check here). These certification will be automatic upgrade to VCP6-DTA and VCP6-CMA respectively without taking another exam.
Do note that this does not renew any of your VCA if you own any. VCA is just an accreditation for individual in the business function to prove they understanding the VMware solution they acquired.
The main jest of it is here upgrading to VCIX6. Reminder VCIX6 contains two exams: Design & Administration similar for all three tracks: DCV, DTM, CMA.
For existing single VCAP5 either in Design or Administration, you will need to take the other of what you do not have in VCIX6 exam. E.g. if you hold VCAP5-DCD, you will need to take VCIX6 administration exam and vice versus. This applies similar to all tracks.
For existing dual VCAP5 for a single track (Design & Administration), you can choose to take either VCIX6 exams; design or administration.
For existing VCIX-NV, you will be upgraded automatically to VCIX6-NV.
So to take VCAP design or administration first, before doing one of the other exam of VCIX is an option or is it better to head directly to VCIX by taking both design and administration exams is subjective. One give you a VCAP certification upon passing while the other needs both exams to be VCIX.
One thing to note, each exam in VCAP is 4 hours each for design or administration while each exam in VCIX is 2 hours, taking both adds up to 4 hours to achieve VCIX.
An incentive, while passing VCIX, it will automatically renew your VCP6.
For VCDX5, this applies the same. They are only required to take VCIX6 design exam portion for the tracks they are on. This also renew your VCP6.