This story is a slight AU. The largest change is that Catelyn, after her marriage with Ned, did not stay in Riverrun for the duration of the war. Instead, she was sent up to Winterfell to get used to running the castle and to assist Benjen, who was around 14 at the beginning of the war.
As a result of this change, thanks to the cold 'summer snows', Catelyn got sick just before she gave birth, and had a high fever after Robb was born. Then, mysterious cosmic coincidence swaps her soul with that of the OC.
I'd like to point out that the OC is an amateur history buff, but comes from a universe where A Song of Ice and Fire was never created and/or published. As such, he has zero knowledge of the plot of the story, or of the characters unless and until he meets them. The knowledge of history helps offset this specific knowledge, but don't expect his knowledge to be the silver bullet that will save Westeros. After all, he'll need to figure out how to actually make stuff and put it to good use.
I couldn't adequately portray the OC's initial panic and adjustment to his (her?) new circumstances, so I elected to leave it out. That's not to say that all the issues have been worked out in the background, it's just that the worst of it is in the past. Other issues will crop up moving forward, such as the one hinted at in this chapter. These adjustments (and the butterflies his decisions make) are one of the main points of this story. The other is the gradual introduction of whatever technologies he can practically recreate. So don't expect steam engines and trains in less than a decade; that's just too far out of reach. But simpler technology, like hand-cranked spinning machines and the addition of the flying shuttle to a loom? That's within reasonable reach.
I have several chapters written already, but I will try to hold onto them as a buffer, in case I run into writer's block.
Hope you all enjoy!