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.
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, you can choose to take either VCIX6 exam; 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 better than 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 and administration while each exam in VCIX is 2 hours, taking both adds up to 4 hours to achieve VCIX.
For VCDX5, this applies the same. They are only required to take VCIX6 design exam portion for the tracks they are on.