You guys have me stumped.
I am a registered building practitioner in restumping and underpinning, and i can tell you now, you cannot determine the condition of stumps by digging 100, 200 or even 300mm from the soil surface level. Hence the thought of replacing a hand full of stumps is rediculous. The first point of deterioration occurrs primarily in the sole plate first. Also, a stump deteriorates from the ground up, not from the middle down. I can tell you that if you have more than 2 or 3 stumps deteriorated, I could guarantee that more than 80% of your stumps have deteriorated. I'm not here to promote business, i'm just here to tell you it's an urban myth to think you can get away with replacing a few stumps. Do you drive your car with three wheels?
My email is [email protected] for professional advice, which is free.