Pre-installed NIS 11 on new computer - how do I download existing subscriptions instead

I have two computers, each has NIS 11 installed (as updates) and fully subscribed. I have the keys for both products (but not CDs). For each of my two accounts, I can install on a further two computers by download. I have now bought a third computer, that comes with pre-installed NIS 11 and a trial period. I have not run this yet. I do not wish to have a third acct and would rather download one of my existing accts instead of activating the preinstalled version on the new computer. How do I do this? Do I remove the pre-installed trial version on my new computer, then download one of my existing copies, or if I run the pre-installed version on my new computer, do I have an opportunity to add a licence key, instead of having to put in my credit card card details for when the trial period expires. Thanks for the help