There's a mysterious fab entity known as "the Recipe" - the product of long iterative dialing in of the fab's operating parameters. Of which there are a great many. A modern fab performs hundreds of manufacturing steps, with thousands of tweakable parameters, and they may interact in non-obvious ways to affect the outcomes. This is what's discovered and adjusted as the fab runs.
The difference between having the Recipe and not having the Recipe is the difference between 96% yield and 12% yield.
Changing the process (i.e. 4nm to 2nm) is the most sure way to lose the Recipe. The fab knowledge you spent months and years of engineering work discovering will no longer apply. But you can also lose the Recipe by replacing fab hardware, by changing the suppliers, by an act of god, and more.
My head annoyingly refuses to accept the non-deterministic outcomes when applied to electronics. In other disciplines, let's say bow or musical instrument making, you need to harvest the wood, age it, dry it, pray to a deity, work with its natural im/perfections etc, but this is "just" bits.
The unfortunate truth is, "bits" sit on a vast foundation of chemistry and physics pushed to their limits. It's a (mostly) deterministic subspace, carved carefully into the non-deterministic world with sheer effort and skill. This is why those fabs are so expensive and complex. Nature defied, reproducibly and at scale. Keeping the Crawling Chaos away one wafer at a time.
If one line is performing well, you can copy its parameters to another, as a starting point. It'll get you some of the way there. How far? Have fun with that. Those two lines, you see, are not the same exact line copied twice. The equipment is the same, but our "the same" is a short for "merely very similar". There are subtle differences. There always are. And some of them matter.