900$ Trany rebuild?

Since I had my trany out while swapping motors I thought I would have it checked out by a local trany shop. I expected a clean bill of health but instead he called and said it was Toast.... seemed to shift well but I was having issues with kickdown. They said it would run 900 including a new Converter... this one only has a few hundred miles on it. He says it is contaminated from particles but I never noticed much debris in the fluid . In fact it always looked super clean.
Cant I just flush the converter ? It has a drain plug...