Analyses are rarely right first time — you see the answer and realise you asked slightly the wrong thing. Modifying keeps everything that was already correct and changes only what you want changed.
Because you would lose the comparison. Rebuilding from scratch gives you two answers with no guarantee they were produced the same way — modifying gives you two answers that differ only where you intended.
Tell us what went wrong, in your own words. We capture the page you're on automatically, so no need to describe where you are.