The brz headlights ARE the same dimensions as the FR-S, so you are able to just swap with no hassle.
Here's my
reference for the headlights.
Also, if you want the HVAC and ac-control unit from the BRZ as well, one thing to note is that the BRZ
limited trim (priced at $27,495) is the one that has this ac-control unit:
The premium version (base model priced at $25,495) has the SAME ac-control unit as the FR-S ($24,930), not the one depicted above.
That being said, we just have to wait and see if just swapping out the parts will cost less than $2,565 (the cost difference of an FR-S and limited trim BRZ), which I'm sure it'll be less. I called up my Subaru dealer just minutes ago and they don't have a parts catalog yet.
I'm looking to do the exact same thing you are, lol.