>Before that, 0/2 levels were very high.. Allowing animals to grow so large.
It wasn't just the Oxygen in our atmosphere that allowed creatures to get much larger. Those creatures would not have been able to support their own weight under the force of "gravity" we observe today. The Oxygen levels decreased after the Great Flood, because it was likely a nova flare that ejected a bunch of Hydrogen, which mixed with the excess Oxygen in our atmosphere, to create an abundance of water that rained down "from the heavens" for a significant period of time.
>The question was, the living 900yrs before that, what changed?
The moon crashed into our atmosphere, split apart the main landmass, started Earth's rotation, "beginning time", which was initially based off the moon, since our planet had yet to establish it's orbit around our present sun and things were quite chaotic for a brief period during the transition. So, if someone marked down 900 year lifespan, it probably didn't have much relevance to our modern year. If these "years" were based off moon cycles, and there's approximately 12 moon cycles in a year (strictly for ease of calculation, then 900/12 = 75. Not outrageous for someone to live 75 human years, if that's where the misconception may lie. Just something to think about