Simple Method:
1. Add new CD-Key to registry where original key was, using a cd-key changer.
2. Make registry un-editable by any program (including yourself)
3. Steam can not reset CD-Key and you have new CD-Key
Time from CD-Key to memory: 2-15 minutes.
Harder Method:
Use a CD-Key changer to find what the hash of your key is (How it is stored in registry, it's actually very simple maths/logic) and then put this into the memory of the game using cheat engine. The memory address is static. (Meaning you could write a program todo the same thing)
Time from CD-key to memory: 1-10 minutes.
Hardest Method:
Write a program that hash's the CD-Keys for you and then put these into memory.
Time from CD-Key to memory: 15 seconds.