Nissan altima 2005 after market fob key and spare key reg.

I have a 2005 nissan altima and i made a spare key(chip key) for it from the original key, which costed me 80$(from nissan dealer).

I would like to know about the after market fob keys that exist for the altima like
http://www.amazon.com/2002-2006-Keyless-Yourself-Programming-eKeylessRemotes/dp/B003T0PAFI

The installation procedure like in
How can I program my key fob for a 2005 Nissan Altima? - Yahoo! Answers

says that
3. Insert key on ignition key cylinder and turn ignition key switch to "AAC" position
At this time, the new ID code is entered and original (previous) ID codes are erased.

I would like to know if performing this procedure erases the code in the car and puts a new code into the car, key and fob?

If so will my spare key become useless in this case?

Please let me know. As I dont want my spare key to become useless as it might be having the old code and not function then.

Thanks.