I currently have a TPG ADSL account. What infrastructure and installation support is required to change this to a TPG NBN Plan? Does a technician need to physically access the home and make changes to cabling etc or is everything done outside the building or even remotely?
That would depend on what type of NBN was available in your area.
If NBN FTTP (Fibre to the Premise) was available then there would need to be a Fibre Optic cable run into the home and some equipment installed inside your house and mounted to a wall.
If it was NBN HFC a coaxial cable would need to be run into the home.
For all other NBN types the service would come into your home via the same wires your ADSL service is on. NBN are responsible for this work.