translation - Difficulties understanding one sentence

ufotableが書き上げた脚本を、奈須きのこが原作者としてリライトしたのが本稿である。

My question is according to this sentence what happens to the written text of the film. Sorry for my low japanese knowledge but I started 2 months ago.

Edit (this edit was made after the answer had already been given): I found another sentence which is more relevant as this time it is said by the author himself in his blog

http://www.typemoon.org/bbb/diary/log/201506.html

#25の制作は「ほぼオリジナルなので、まずは原作サイドで書くべし」と始まりました。
そしてできあがったきのこによる脚本モドキを前に頭を抱える制作陣。

It was answered (to make sure) here: Understanding the true meaning in this sentence

A good translation would be:

The production of episode 25 began with "since it's mostly original content, it should first be written from the original author's standpoint". And then the production team clutching their heads at the completed so-called script by Kinoko.

So basically, as Naruto says at the end of the comments in that question: The sentence in this question says "Kinoko wrote it first as an original work, and ufotable modified it"

Answer

「ufotableが書{か}き上{あ}げた脚本{きゃくほん}を、奈須{なす}きのこが原作者{げんさくしゃ}としてリライトしたのが本稿{ほんこう}である。」

Two different works are mentioned here:

1) ufotableが書き上げた脚本 ("the script written by Ufotable")

2) 奈須きのこが原作者としてリライトしたの ("(what/the thing) Kinoko Nasu has rewritten as the (original) author") 「の」 is a nominalizer.

And please note that the subject of the sentence is 2) and that 2) is based off of 1).

"This is the manuscript where Kinoko Nasu, as its author, has rewritten based off of the script written by Ufotable."