Replacing a DDR2 memory with another that is faster
I assume you mean replacing an IC on a board, not replacing a memory module.

If that is the case, and provided the parts meet the DDR2 standard (every part I have ever seen does) then there should be little difficulty provided you intend to run the part at the speed of the part you are replacing. That said, One particular check to make is that the inputs and outputs properly meet the SSTL_1.8 requirements.

Most timing parameter issues are with the controller rather than the memory devices.

It will be prudent to re-check the timing parameters, but I have done this (due to parts becoming obsolete as slower parts simply do not sell) on numerous occasions with no problems at all. 

There is no reason at all that a part rated for 800/1600 operation cannot be run at 400/800 speeds; just make sure the correct CAS value (in particular) is loaded at DDR initialisation. I have never had to change a DDR memory side parameter when replacing old parts with higher speed capability devices.