That is not an upgrade path, or at the least, a very poor one, since none of those CPUs show much in the way of a performance increase over the 8400. CERTAINLY not enough to turn around and spend another 400 bucks later for a CPU that might net you, maybe, an overall 5% average performance increase in applications, most of them anyway, and practically zero gaming performance difference.
That's more like a side grade than an upgrade. Sure, in VERY optimized game or application source code, you might see some larger differences when those extra threads are used, but that is STILL to this day vastly outnumbered by the number of games and applications that do not, or at least not beyond the six cores it already has anyhow.
Sure, there IS someplace to go from where it IS, but the distance is certainly not worth the cost of the journey. Usually when we talk about an upgrade path that is within the same generation, we are talking about like from a Pentium or i3 up to an i5 or i7, not from one six core to another six core that happens to have a few hyperthreads tacked on. Factually, I've not seen ANY really consistent evidence that more than six cores shows any benefit in ANY gaming benchmark aside from ashes and civilization. Might be a few small differences here and there but mostly, there's not. So, I guess it's a matter of what you feel your four hundred bucks ought to buy you later.