Doors and Windows - Rotted sloping sill Pella Double Hung

A few of my 30 year old Pella double hung windows have enough sloping sill rot that I can push through the wood. I have removed the decorative trim on the bottom and notice that the bottom wood of the window itself needs to be replaced. The adjacent edge of the plywood is also a little flexible. The overlying wood siding is in great shape with no premature paint peel or cracking.
My question is, can I cut out the lower sloping sill and window
wood and replace them without having to replace the whole window? Should I pull off all of the adjacent siding to evaluate the plywood?
Dave

There is a statement that I have never heard, "rotted Pella components". I get the feeling that Pella engineers never heard of wood preservatives.

To answer the question, those sills can probably be repaired, but at that age, I would put the money into new windows, and not Pella.