As we near the July 2017 Update for Dynamics 365 it looks like an option for early upgrade has shown up for portal add-ons.
You can find this option on new and existing portal add-ons in the CRM Administration Center under Applications and Manage your Portal Add-On. This option should be used with some caution as it will mean you are in the first release program for portals. What will you get? The next portals release! That release previously was looking to be version 8.3 based on all the information provided at eXtreme365 and the other information available, but it is looking like we might be making the jump to version 9.0 sooner than expected. The roadmap items are still the same as published on roadmap.dynamics.com.
Here is what the new option will look like in the portal add-on management:
Check this box with caution!
Update 6/23/2017: Correction to the versioning, as earlier expected the portals released now will be v8.3 not v9.