Can I safely increase the number of processors on a VM?

Solution 1:

Increasing the number of CPU from 1 to anything more is safe on modern versions of the OSes/Distros you mentioned. Modern kernels dynamically adapt to the number of CPUs at startup and have no problems with you adding more. Old versions of Linux and Windows (Around 2000 and before) do not adapt well to change. The same is essentially true for reducing cores; though you also run the risk of bottlenecking the VM.

Solution 2:

The answer is the same as the question that you just linked it.

In OSes that have a multi-CPU kernel/HAL enabled, this isn't a problem. In a Windows XP/2003 or earlier VM with a single CPU HAL installed, there will be a performance hit. I've never seen actual instability from this, though.