1. I waited until the almost the end of my monthly subscription period with my old Substance Indie subscription.
2. I took out a new 3D subscription direct with Adobe. They offer two variations the 3D texturing subscription and the 3D collection subscription. However, there was a substantial discount for existing Substance subscribers, subject to a time limited offer. I received that offer by email direct from the Substance team and so I used the subscribe link for the 3D collection in that email. In short - if you are an existing Substance subscriber - check your email.
3. I downloaded the Adobe versions of the apps from the CC desktop app and un-installed the older versions.
4. Once the subscription was up and running in my Adobe account, I cancelled by Substance subscription.
I still have access to the Substance Source points and materials I built up under the Substance subscription, by logging into the 3D assets site. At first this shows the points from my Adobe subscription, but by logging out of the assets site and logging in again using my old Substance credentials my old points and materials are visible and usable. Adobe have stated that they are looking at a process to transfer those assets to avoid that double log in, but it does work in the meantime.