Yes, it's only software.
Again, the HW part is in the car if you want it or not ( yes, of course, even the mobileye part.. that of course now will be built by tesla or other, this doesn't matter ), you need ALL the HW present for apply all the security features, it has 0 sense to add 4 camera ( just to say a number ) for a car wich has autopilot and toggle it to a car that doesn't have it.
It create complexity for the software, it create complexity on the line, and in the best case enable you to keep in hand something like 50-100$ at the cost of re-wiring all if someone whant the autpilot added later, so, again, the autopilot upgrade is purely software.
Of course you need the HW part to make it, but you don't need HW changes to enable the autopilot ( like in the current version ).
Yes, of course they can change something in the future release of the autopilot v2, and yes of course we aren't tesla employee and even if we are there is no guarantee that the final version is the same as what they are developing now, maybe they discover that they need another camera, or a lidar or.. who care?
But you don't need to be to give a good enought estimation based on the fact that we know and based on common sense.
And this is of course for the autopilot part.. for the audio part.. better audio = better HW.
you can of course downgrade the HW but it's absurd and it's never done, usually a better HW need more space and it's heavier, so the only way that an upgrade on the audio part doesn't need HW changes is when there isn't an audio-upgrade options.
Or the upgrade enable spotify pro vs spotify free, but in this case you don't have a better audio-system, just better tracks to play