I am fairly sure (but not 100% sure) that Tesla offers computer upgrades from 2.5 to 3.0 separately, for like $1500 or something (speaking model 3 specific, now). I think the intent is for people who have HW2.5 to get to HW3.0 so they can subscribe to FSD.
I dont know what the warranty on that upgrade is, though.
EDIT: Googling to check if I was mis remembering this, turns up a few articles that confirm it, and state tesla dropped the price to $1000.
Less than a week after going live with its Full Self-Driving subscriptions, Tesla reduced the price of its Hardware 3.0 retrofit offer to $1,000. Hardware 3.0 upgrades was previously priced at $1,500 per vehicle. Unveiled in April 2019, Tesla’s Hardware 3.0 is a custom-built Autopilot computer...
www.teslarati.com