Eric Malund is correct. So is Michael Karas. The short answer is that in general you can NOT replace a faster version of the same chip and be assured that it will work. As Mr. Karas pointed out, there are issues of timing. Will all of your setup and hold times still be valid? But more important is the question of rise time.

First, stop thinking of the speed of your circuit as the clock speed. That only dictates how fast your microprocessor performs operations. Electrically, with regard to the board traces and the components they interconnect, the speed is dv/dt (change in voltage per unit time). Assume that in a nano second the electrical signal travels about 6 to 8 inches. If your signal changes from low to high in 2 nano seconds, and your trace is more than 16 inches long (the critical length = 2 ns * 8 in), then there will be a time when components on the same net will have different voltages on them. This can really mess up your timing. It also sets up reflected signals between improperly matched lines and loads. The physics of high speed circuitry is a field of study in itself. The point here is that if your traces are 10 inches long, shorter than the critical length, then you don't have a problem. But, if you switch to a device that has a 1 ns transition time, now your traces are longer than the critical length. Now you can get reflected signals creating false switching, or perhaps your board will start spewing emi.

This is in general how switching to a faster device can foul up the works, even if you keep the crystal frequency fixed. Check the dielectric constant of your board, find out the new switching speed (transition time), and calculate the new critical trace length. If your traces are still shorter than that, you'll PROBABLY be okay. You will still need to test the new configuration. But if your traces are longer than the new critical length, then you might as well send it straight to your board designer (and hope he understands the high speed issues you have).

There's my two cents worth,

Good Luck