Hello and welcome to the forum.
Have you seen the iFixIT page for the FP3/3+?
There you wil find a teardown of the phone.
Unfortunately it is not possible to switch the SoC (processor) of a phone. In this behalf a phone differs a lot from a computer.
With a phone the OS is something specific to the phone and it’s configuration.
E.g. when changing the camera-module of the FP3 for the “plus” module of the FP3+, it is not possible to just install a new driver, but you have to upgrade to Android 10, that does include the drivers for this camera. If you would switch the SoC, you would have to program the OS anew. No manufacturer can or will do this.
Besides that, you would have to do the soldering, which would require a special tool. A standard soldering gun won’t do the trick.