Apart from the arguments already brought by other people (energy use, reuse rather than recycle, etc.), I think this idea would also be very difficult to realize from a logistics/business standpoint. Presumably only a limited number of FP2 users would trade in their old FP2 for a new FP3 (particularly with this particular group of buyers, which is very longevity- and sustainability-oriented) - this means that Fairphone would have to design and implement two completely separate production processes. One that involves refurbished/recycled parts from the FP2 (which also have to be separated, tested for functionality, etc.) and one that builds an FP3 from scratch, without any prior parts. Not to mention that for the first model, the parts that can be reused might be different from each old FP2, so it’s not like you could standardize this process very well.
Also: what kind of warranty/guarantee would Fairphone be able to give on those new FP3s, if some of their parts are from old FP2s? And how many buyers would be willing to buy a half-new, half-refurbished phone with either no guarantee or individualized guarantees for every individual part, based on whether it’s new or refurbished…
I think this would be enormously difficult to realize, personally. Given how challenging the production of the FP2 (all from scratch, but modular, fairer and in small quantities) has been and how much frustration it has caused among users (due to bugs, delivery delays, etc.), I don’t think this would be a good path for Fairphone to take at this point.
What I could rather imagine is that Fairphone takes back old FP2s (in general, whenever people stop using them) and has them recycled properly and then tries to feed some of the recycled raw materials back into their production. But this is a lengthy process and can’t be done one-to-one, old FP2 to new FP3. Plus I don’t think this is what you meant to suggest.