First you have to decide if you want to change the hardware that is installed in your door or just change the key that works your existing hardware. The term re key refers to having a new key work your existing lock hardware. Usually you will re key your locks if they are working fine but someone has your key that should not have one (i.e. You've just moved into a new home or your keys were lost or stolen). You would usually change your locks if they are worn out or you are upgrading to a higher quality lock.

This is called having your locks keyed alike. First the locks have to be all from the same manufacturer. Although some keys from different lock companies will slide into another company's locks it is not advisable to key different lock company locks alike as the pin to pin distances are usually different.

No. Locks keyed alike have one key that will work them all. Master keyed locks have more than one key that works them. In a typical master key system you would have a separate key that turns for each lock AND one key that works all the locks.

Yes, if the locks are construction keyed. No, if they are master keyed. The only way to be positive a contractor's key will no longer work is to have a professional locksmith re key the lock and drop out the master pins.