Once you upgrade your VPS the RAM and CPU are easily upgradable, with the disk it's a different matter:
With OpenVZ your disk space will be upgraded using the same partition.
With XEN Linux we can also upgrade it using the same partition.
With KVM Linux, we have to add a new partition using the extra disk space.
With KVM Windows, once the upgrade is completed, we can resize it via the disk management feature under the windows tools.
