1. You're OK - VCP-DCV 2019 came after VCP6-DCV.
2. Selecting the "VCP6-DCV or newer" option you'll see that the training is "recommended" for you, so it's an entirely optional step - in other words how you choose to prepare for the NSX-T exam is up to you.
This is where course and exam announcements are made: VMware Education Services
No mention on there yet about exams based on NSX-T 3.0.
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