Using Converter I can easily change the number of VCPU, virtual CPUs.
If I change the number of CPU for a Windows Server shoud I reinstall or reconfigure the guest Windows Operating System (let's say Windows Server 2003 or Windows Server 2008) to make it aware of the new number of CPUs?
In other words,
I'd suggest you make sure you are always running the OS with the correct HAL. Usually the newer OS's automatically pick up the multi-processor HAL when you add a CPU, however they don't revert to a uni-processor HAL automatically when you remove a CPU.
see e.g. http://kb.vmware.com/kb/1003978
After some investigation from the Microsoft point of view it looks (I'ts my personal opinion, I can be wrong!!!) that Windows Server 2008 allows increasing or decreasing the number of VCPUs without the need of any configuratiion change, while older versions require some additional chach: is it true?
From Vista the HAL is generic and you can change the number of vCPU without change anything in the OS configuration.
The problem was in previous version, expecially to go from a multi-CPU HAL to a uni-CPU HAL.