The only thing we know that was supposedly changed from the original script was The Origins of The Turtles. For all we know everything else was kept. We just dont know enough with this movie
As someone who has read the original script, and has followed this film VERY closely since the delay and script rewrite, I can tell you with full confidence that this is a completely different film.
At the LEAST, 90% different.
For example, Vernon Fenwick, Channel 6 news, and the rest of the Channel 6 news crew were NOT in that original script. Hell, in the original script, April didn't even have a real job reporting. She was an unpaid intern at CBS (gotta love brand placement), and the only time that "job" was relevant in the script was to explain A) the issues in her and Casey's relationship and B) to serve as a plot point and location for where the Turtles find a portal to Dimension X and have a cool scene where they evade the Foot.
But let's move on, and talk about the script exclusions some more.
Also not in the Blue Door script are Karai and Baxter Stockman. Neither of those 2 are to be found in that script, but are said to play considerably large roles in the new one. Also different is the Shredder who, while still largely different than what we've seen before, is said to be much closer to previous adaptations and the source material, than the "Spiky Alien From Dimension X" version that we saw in the Blue Door story.
Now, on the flipside, there are some very notable characters that were featured in the original script, that are no longer present.
Bebop and Rocksteady were featured in considerably large roles in that script, having a fight scene with Splinter in towards the middle of that script, and then being present in the climatic battle between Shredder and The Foot; and Splinter, April, The Military, and Casey Jones.
And while on the subject, lets not forget Casey Jones, who was arguably the main character in the original script and had as much screen time, if not more, than April, let alone the Turtles. And where is he now? Completely gone from the script.
There's also elements such as the clothing, Donny's gadgets, and the Turtle Van, which all were not present in the original script.
Also, there are scenes that have been filmed (such as the infamous trampoline scene and the Times Square scene) that were not present in the original script.
Point is, there has been a VERY noticeable revamp to this film. Now that's not to say there aren't some retained elements, because there are, such as the militarized foot (whom do sport a traditional look at points in the new story). However, much has been changed. Enough to consider this a entirely new film.