If that's not our prerequisite, our entire discussion up until now will be changed.
If that part of the story is different, then our whole discussion until now will all change.
ご質問ありがとうございます。
① "If that's not our prerequisite,"=「それが私たちの前提でなければ、」
"our entire discussion up until now will be changed."=「今までの私たちの議論はすべて変わってしまう。」
② "If that part of the story is different,"=「話のその部分が違うのであれば、」
"then our whole discussion until now will all change."=「私たちの今までの議論は全て変わる。」
ご参考になると幸いです。