The bumper mounting points are identical for
BRZ Preface - BRZ Facelift - GT86 Preface(=FRS) - GT86 Facelift,
meaning there are 16 different front end conversions you can do if you count front bumper only.
However, as the headlights (and fender garnishes) are also different the possible combinations go up even more. This takes too much time to get into and is up to the user.
Why the interchangeability?
Economics teaches us that a Facelift usually serves the purpose of keeping product sales up or increasing them, effectively extending the product life via purely cosmetic means.
That's a good way to make more money off the old product before introducing a new one.
So basically, making the parts not interchangeable would defeat the entire purpose of the production, which is laid out to be as modular as possible to keep costs low. Especially with a joint Subaru+Toyota project, this is paramount.
But a manufacturer isn't stupid. They know people will try to swap parts around and over. It's been done before. A lot.
Take the famous Nissan Sil80 or 96-01 Civic.
So they try to make things a little harder by purposely changing some details between models to make the switch harder. They want people to buy the entire facelifted product, not just buy the changed components and bolt them on easily.
For example, the '88-'91 Civic/CRX were so similar that Honda swiftly changed the mountings of the seat just so people couldn't drop the more desirable CRX seats into their cheap Civics.
I'm sure you've noticed that I'm not only talking about
Preface-Facelift changes anymore.
But this is an important primer.