I don't think so.
I don't know what you rly mean im noob but,if its as i think,then my answer is: i once disinstalled then instaleld with new serial cod4 ( cracked ) and i didn't lost anything from my acc
First off ...
If you change it in Regedit you will loose your Rank
If you change it in Multiplayer Options only that profile on that you changed the Key in these Options will have still his rank (I know what im talking about )
AND : @rowan112 : No if that key isnt banned on that Server (i think he is playing cracked servers) he will be unbanned