Hii!
If you sit for an improvement exam and get better marks than the previous time, you will get a new marksheet with the improved marks printed on it. So, there's no point of previous marksheet being valid or not, because you would always want to present the marksheet with better marks in any exam or interview as such. If by chance you don't improve, the best out of the two marks will be taken into consideration.
Hope this helps.
Good luck!