Not an upgrade, definitely a downgrade, the 2500K is slower than I7 920 in most applications including gaming. If you want an upgrade you should consider 2600K but first you need a better GPU.
In many games that aren't highly threaded (and there's many), even the Core i3 2x00 is sometimes faster, or right on it's tail, in gaming, and that's a dual core!
It does this while also yet using less power.
That's before factoring in higher overclocking potential.
How does all of that make Sandy Bridge a downgrade from any quad core LGA1366 chip? It's not really worth moving to from it, no, but it's still no downgrade. It's not even a sidegrade. It's a clear upgrade.