If you are going to upgrade and haven't got a soft sampler yet, then upgrade to Sonar 3P ( this includes the Vsampler) during September and you will get a free upgrade to Sonar 4P ( which comes without it ). This according to Cakewalk.com/forum and I think Edirol UK.
I think you may have trouble getting hold of an upgrade copy to Sonar 3 now. It's no longer available, otherwise people could do what you are saying, and get all the features of Sonar 3 effectively for free. (Eg VSampler)