At Post C2+ Splashdown Briefing - May 31, Musk said
..first small technologic upgrade of Dragon no.6 (SpX-4). Next big upgrade of Dragon v2 (reusable, with motoric landing by SuperDracos).http://www.space-multimedia.nl.eu.org/index.php?option=com_content&view=article&id=7628
I don't see other reasons for upgrade of Dragon 6, then by reason of using him together with F9v11. I mean upgrades like enhanced storage of fuel in Dragon capsule, extended trunk, etc. Let's say Dragon v1.1.
There is possible that Dragon 6 will be first on F9v11 because of that, but in parallel it doesn't mean that Dragon 5 (Spx-3) can't already fly on F9v11 still in non-upgraded version in terms of progressive changes.
So my estimated scenario:
SpX-2 - F9 flight 5 - Last F9 v1.0
MDA Cassiope - F9f6 - First F9 v1.1
SpX-3 - F9 flight 7 - First using F9v11 for light version of Dragon
SpX-4 - F9 flight 8 - First upgraded Dragon(v1.1) for utilize full F9v11 capacity