I do think looking at the past could give us a bit of a clue to what this 'big upgrade' is.
If anyone remembers, PSU's PC and PS2 servers were shared in the same way PSO2's shared servers work. Due to PS2 limitations and needing to use a memory card as the storage medium for patch updates there really wasn't much that could be done in terms of content updates and this in turned affected PC updates.
When SEGA JP announced they would be ending PS2 support they also announced a major update coming to PSU... sounds familiar? Of course, this major update may not seem like much nowadays since its the standard PSO2 one; New Areas, New Weapons, New Costumes, etc as well as the game transitioning to a Free-to-Play revenue model.
It certainly makes what kind of major update PSO2 gets after Episode 6 interesting but again it all needs to work within the game engine's limitations. PSO2's engine was developed in 2008 and has large portions based off of PSU's engine... how far you can get with an engine that old is something to keep in mind.