Although this is thread is in the Post-FIRE section, the OP clearly states "FI", not "FIRE" or "RE".
I've been FI for several years, not even sure exactly when I first hit it because my goal was always to save far more than that. At the market high in Sept. of this year, I hit 3X FI and am still around 2.8X FI since the recent drops.
What changed? Really, nothing at all for the first few years of being FI, and then I started thinking, planning, and talking more about FIRE over just the last couple years of being FE as my stash grew to a more desirable level. I've always had a high savings rate, but I've actually tightened up the spending even more since I've hit FI, although being FI was not the reason for the drop in spending, but rather a change in my feelings about what seems like wasteful spending, even though I was already pretty frugal to begin with.
If the question was about FIRE, I couldn't quite answer yet, but in the future, I imagine I would have a lot more to say about the significant changes in my life.