Its highly likely that collins will need to be cut open as part of the LOTE. It looks like effectively being gutted.
The LOTE is going to be quite extensive, including changing the diesel engines, the main electric motor, electrical distribution, sensors, computer systems, and all the normal maintenance, pipes, hvac, batteries, waste, etc. Effectively putting all the stuff into the Collins hull that we wanted in the Attack class.
Could it be made bigger (longer with some sort of hull extension)? That currently isn't part of the scope. In past history, in different countries, some submarines have changed dimensions in such a reconfiguration to fit additional capabilities or equipment. TBH detail design AFAIK has not yet started, so we won't yet know.
However, Collins, with brand new, more powerful and reliable MTU diesels, new more efficient and more powerful main motor and generator, new sensors, new subsystems, latest combat system revision, new batteries possibly of a new battery chemistry or type, would be an impressive diesel submarine. I would imagine they would be heavily looking at sub systems from the Astute/Virginia class to see what can be incorporated, to both improve Collins and have commonality with the future submarines. Also the supply line for Collins is right now.
While there is a lot of singing and dancing over the new subs, our existing subs are actually the platforms we have, and the first six submarines we actually "make" and that will make up the majority of the sub force for the next 20-30 years will actually be Collins.