Some chipsets are compatible enough for you to keep using your existing OS.
I have moved XP on a friends PC from VIA KT266a to KT133 to KT400 MB's without installing. She is still using the same OS I installed for her in 2001 or 2002.
I had similar luck between NF4 and NF2.
Also repair install sometimes will let you keep using your existing OS.
However if the chipsets are not identical then you often have some performance problems. My friend's KT400 system scores 25% lower than it should on memory bandwidth benchmarks, despite my having reinstalled the chipset drivers.
Really its best to do a clean install.