I'm on my third pacemaker, and so far there has been nothing said in this thread that I can disagree with. Whether or not they keep you overnight will greatly depend on how you react/recover in regards to the anesthesia. The one thing you can expect is the new pacemaker to have more features, such as being able to connect and download via your mobile phone instead of using a dedicated external device you place over your pacemaker. If your current pacemaker doesn't already use this, the new one will most likely have a phone app that will let you see how it's doing, time left on the battery, log any issues you encounter, transmit a report to your doctor, etc.
If you already have existing wires, your new pacemaker will use your old ones. I had an additional wire added after I broke one at work right before the changeover to my second pacemaker. By the way, they don't remove the old ones but rather just abandon them in place.
The very newest versions of pacemakers don't even use wires at all - the generators are super small and are emplaced directly onto the surface of your heart muscle. They are supposed to be much easier to install and replace, although I don't have that type myself.
Good luck and feel better soon!