For Infinity, an engineer would visit and replace the current master socket with a new filtered one. The new Home Hub must plug in to that master socket, not an extension. The installer is supposed to offer to fit a data extension lead if the master socket is not in a convenient place.
The installer would also visit the local green roadside cabinet to switch you to Infinity.
There are no free upgrades to YouView.
If you want to watch the new channels streamed over broadband, the Vision box is supposed to be connected directly to the Home Hub via an ethernet cable, rather than powerline adaptors. That said, many people on this forum report that powerline adaptors still work fine for them.