It's pretty cool that Stihl has engineers who are constantly seeking improvement in previous designs, and addressing possible issues with current designs.
I know when I went to the plant, they spoke about design improvements, and the inherent constraints they must deal with.
They seek design improvements when they see an increased demand for certain replacement parts.
This means that failing parts need an improved design, and must fit earlier serial numbers as well, if at all possible.
A last resort is a major design change, so there either are great benefits, or a flaw in the existing design.