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2L here. I was slightly below median at my T14 after my 1L year. Managed to land an SA at a V100 because I have a strong resume. The only thing holding me back was my grades. Last semester I got top 1-3% grades and even "booked" a few classes. If I can do even half that well this semester, I should be able to to pull myself into the top quarter of the class.

Do I have any chance at upgrading to a better firm if I do?

What is the best way to go about doing this? 3L OCI?

Yes and yes.

Just keep in mind that vault rankings don't really mean that much on their own. Moving to a firm with a better culture, stronger practice in the area you're interested in, more desirable location, better financial stability, etc. can be smart. But don't blindly chase "ranking."