There were some prototypes like phonebloks (later named project Ara) ten years ago, which sadly was cancelled
Interchangeable parts between models is really hard, since your support burden kind of explodes when you have to support several parts for several models, so it would probably not be sustainable for a smaller company like Fairphone
With that said, I remember a Fairphone representative saying they were looking into if they could provide upgrades for the FP5 down the line, as the longer lifespan of this model might makes it economically feasible for them
I wouldn't base any purchase on that until there's an official announcement though
Personally I'm happy enough with the value proposition as it is to be getting myself an FP5