Could the new sub be a tech tester/demonstrator, perhaps for next gen SSN tech, rather than being the first of a new class of yuan SSKs maybe?
Doubtful.
This is a hull which is clearly derived from the 039A/B family and very much a full sized proper SSK, and is equipped with certain subsystems (like a towed array sonar on the tailfin) that we've seen on other recent PLAN submarines as well.
More importantly, I can't see any technology that this thing has (or which it likely may field internally) that would be exclusive to an SSN that would require testing on a dedicated new test submarine in the first place. Hydrodynamic advancements, general quietening technologies, new sensors, combat management systems, weapons etc are all things that would benefit a new production SSK class as much as a new production SSN class.
And the things which
are SSN exclusive (like a new reactor, perhaps new propulsion arrangements, perhaps a new hybrid hull that we've heard about), are not things that we've had any indication of being integrated for this new submarine nor are they necessarily things that would require testing on a dedicated submarine of this size either.
But perhaps the most definitive factor to keep in mind, is that we've been expecting 039C to emerge now for years, and this new submarine has been referred to as that by the Chinese PLA watching community.
Personally, I'm surprised as to how this submarine could be considered as anything
other than 039C, given our long anticipation of a new SSK class of that name, as well as its obvious heritage from the 039A/B line.