For instance, maybe a chip goes out of manufacturing, and they can no longer get it supplied, but a successor product from the same supplier meets all the same requirements.
Perhaps a bracket goes out of manufacture, but an almost identical bracket is sourced from a different supplier.
This would qualify as a hardware modification, but is a non-breaking change.
A good example is the company that changed the design of their guard-rail endcaps. They reduced the size by an inch, which allegedly resulted in 5 deaths (court cases are ongoing). They (again, allegedly) failed to test the change, and failed to report the design change to NHTSA and the various state highway departments that specified it's use.
Another example is the ignition switch in GM cars. The engineer changed the design, but did not issue a new part number for it. Externally, same shape & mounting points & connector pinouts. Internally, very different.
So while a design change in a part might be "like for like", it requires a full lifecycle of testing and documentation. And that's expensive, so auto makers try very hard to get their designs correct up front.
Tesla's processes will be weird in 50 years - if they're even around. but today, I'd imagine that kind of complexity is way way easier to manage if you're aware of it from the start. Rather than getting it right the first time, build the whole organization around coping with those little variations.
OTOH, i have no evidence they actually do that... Just seems possible for Tesla and very hard for GM.
Traditional car makers do slipstream changes into their cars, but this is just crazy. Think of the service challenges, and the parts catalog challenges. Not to mention the cost of all those different parts.
"Oh, your's was made the fifth week of August, so you need a different bracket. We don't keep those in stock because they're all different. Here's a service loaner and we'll call you when it's done."