Do you mean that you'll be upgrading from a v1 version of VC to a version 2 license?
Either way, I would create a copy of your VC folder and then install the upgrade in the new one (that is, run the installer in the new folder), so that you have a backup if any issues arise.
Re: no. of computers, see "authorized use":
http://voxcommando.com/home/purchase/One of these days, I'll get around to updating our FAQ. No idea why these questions aren't on it yet!