The way I reason this for myself: T/F preference was never clear cut for me, so that wasn't a big change. However, as far as I had developed Fi, there was nowhere to go but to TJ. I have always identified myself as an IN.
Dominant Fi is tied to values/ideals/feelings. It is a judging function. If you find yourself a rational, how can you be an INFP anymore? You can't. It's not just about behavior anymore.
I also have a huge soft spot for INFPs, because I understand.
I should probably go read the Ni/Fi confusion thread now. I used to score Se higher than Si, but I don't know. I wish I could go back to childhood and make some sense of my then self, but I can't. The last time I read Ne/Ni descriptions I identified more with Ni use, although I might be mistaken.
Is there another way to explain all this? Maybe. I'll continue investigating, as far as it interests me.