I had read through the guide. I did not notice any information related to the scenario I described. If you could point me to where in the guide it addresses my question / concern I would appreciate it.
In my opinion you should do it like that (I assume that you have appliance, not windows version):
1) Upgrade PSC Controllers to version 6.7
2) Upgrade vCenter servers to version 6.7:
3) Use converge tool to promote vCenter servers as PSC embedeed
4) Decommission external PSC
A little help:
Of course there is a lot of things which you need to consider. Your plugins etc.
And make a backup! We did such migration and unfortunately we needed to revert vCenter from backup.
I hope I helped.