If forking, what happens to the CASH coin already in people's wallet today?
Nothing changes, you'd still keep it all.
E3A.
Think about when the roads are upgraded or repaired, your car remains the same, it just runs smoother on the new road.
Do I get new tags (registration sticker) for me "car"?
If nothing need to happen, then after your fork, I (or a few of us) can keep running the old software and keep mining with the old pool. I would probably get CASH very fast because most of you guys will have left. Can I spend the new CASH and the old CASH the same way.
There has to be some mechanism to switch over. Or will the new forked network automatically reject new CASH mined the old way (by looking at timestamp)?
A hard fork is just what it sounds like, we fork the block chain.
You set the fork to happen at some point in the future to allow everyone to get the new wallet first, then at a certain block you fork the block chain to make your changes, whether that be changing the algorithm, used for mining, the checkpoint server, or what ever.
If you are still using the old wallet at that point anything you do will only be recorded in the old block chain not the new one.
Hence for a hard fork to succeed you must get the exchanges and pools onside to use the new wallet else it will fail.
So yes you get a new 'registration sticker' that would be the new wallet you need to download