NIS: Buy a Key/Extend the seat count

I currently have 5 PCs on two NIS 3 PC licenses.  (I do realize Symantec sells 5 PC licenses under small business, but it was cheaper to buy two copies of  the 1 user/3 PC license)   I am coming up on renewal, normally that would be simple enough, but I have some additional PCs on order or under consideration, so I'll probably end up over 6, and was wondering what my options are.  I could just renew, which is probably what I will do, but I was wondering what exactly Buy-a-Key did.  Sounds like it might just be another 1 user/3 PC license.  Does the key actually change, or can I use the original key on the additional PCs.

 

[Just as an aside, somebody is probably going to say, having 6+ PCs is not normal, and I suppose it's not, but I don't think it's al that unusual, 2 professional household, each with 1 desktop and 1 laptop, you go over the 3 PC limit pretty quickly, even without factoring in kids (or Macs or Tablets, Phones, etc.) and transition overlaps.  I do wish that Symantec would come up with a model where I could just buy a 1 User/1 PC box, and then buy and extension to the key to add additional equipment or seats.  Then you'd be selling just 1 box per product (not a 1 user/1 PC license (bundled typically), 1 user/3 PC license, 5 user license, 10 user license, etc.).  Tie it all to my Norton account, let me buy however many seats I need.]