pg_upgrade from 9.3.5 to 9.6 with PostGIS (Windows)
PostGIS 2.2 isn't built for PostgreSQL 9.6 on Windows
PostGIS 2.3 isn't built for PostgreSQL 9.3 on Windows.


You have two options


Do a hard upgrade.
Build for 9.6 PostGIS 2.2, then do a soft upgrade


I would do a hard upgrade. Seriously 230 GB is not much to bring in. You'll live. (I think).


  My guess is that it's monumentally unlikely that the entire PostGIS-enabled world can't update their PostgreSQL versions in a reasonably straightforward (i.e., non-pg_dump) way.


They do have a method for doing a quick and easy upgrade, but it requires the server to have both versions of PostGIS. Then you can run,

ALTER EXTENSION postgis UPDATE TO "2.3.2devnext";
ALTER EXTENSION postgis_topology UPDATE TO "2.3.2devnext";


That method isn't readily available to you because I can't find anyone who has compiled PostGIS 2.3 for MS Windows PostgreSQL 9.3, but strictly speaking it's possible.

If you were using Ubuntu/Debian this wouldn't be a problem as a precompiled version of PostGIS is already in the repository for all versions of PostgreSQL also in the repository.