Which EJ207 is it in the car? Version 5/6? If so you will need a 02-05 WRX complete harness merge that to the RS harness. Then you will strip your version 5/6 207 block down to a bare long block and install all the USDM WRX parts. ECU, Intake manifold and all the junk attached to it, exhaust manifold, turbo, downpipe, airbox and turbo inlet. your goal is to make it look like a 2002-02-05 WRX. Then you can stroll into the BAR, get your sticker and then smog it later at and smog place normally/"legally".
My friend da5dime on here just got his version 5 long block BAR'd with the above process. He could give you better pointers on what exactly he needed to do.
If you have a newer ej207 it will be easier because the harness should already be 02-05 wrx and you have all the other parts to make it look like a wrx(unless its a version 8+, then you need to swap the turbo set up to single scroll). A 02-05 WRX ECU will run a version 7-9 EJ207. Like Acetech said its easier if the swap was done with the BAR sticker in mind. I have a version 8 STi swap in my car and I specifically went with the version 8 with the BAR in mind. being able to smog the car normally and be able to get a ref ticket and not be totally screwed was one of the goals for my swap. Also going the 02-05 wrx harness route you have more options for tuning the car as you can to to any tuner and get a open source tune for the car. California is stupid when it comes to car mods :boxer: