Form 7203 and share repurchase from S-Corp

S-Corp shareholder's contract demands that at age of retirement he sells back his shares ratably in four years, 1/4 each year to the company.

Evidently his basis is determined by Form 7203 of the previous year (Line 15.) We just have to take 1/4th of that number for basis. This stockholder basis will change every year though. So what happens in year 2,3,4? Or what we determine now will stand for all the years remaining, meaning 1/4th of his 2024 figure from 7203?

Does it matter that the stocks were purchased at different times at different prices throughout the years? Are there any adjustments to the basis found on Form 7203?

Thank you!