Is my data "re-encrypted" (with FBE) after I change my phone PIN and how long would that take?

I’m on Android 13 and I feel that my old PIN is flawed and can be guessed by people who know me well. I am also under increased risk of my phone being plugged into a forensics device. Question is, if I change the PIN now, how do I know that the data is re-encrypted with the new key derived from my new PIN? I’m concerned that after the PIN changes, the internal storage (/data and /sdcard partitions) can still be decrypted with my old PIN for an extended period of time.

I’m assuming that when the PIN changes, underneath the hood the OS first decrypts /data and /sdcard with my old PIN, then encrypt new files with my new PIN. Anyone familiar with Android/AOSP know if my assumption is correct or not?

There’s no risk, your assumption is incorrect. The data itself is never encrypted with your PIN, there are two keys:

When Android is installed, your phone is encrypted with a completely random master key. That key is then encrypted with your PIN and stored somewhere. When you change your PIN, the master key is decrypted and then re-encrypted with your new PIN, replacing the old one. This happens instantly.

The encryption of the data itself never changes unless you reset your phone.