I have a existing pi set up on my network. I regularly keep a spare cloned SD card ready - I have needed one before today!
I am considering buying a second pi which I would like to be a copy of the existing one but will obviously need some changes to computer name, fixed ip and anything related to the MAC address that will be different.
My thoughts were to use the spare card to boot up, having first shut down the existing one to avoid any network conflicts. My question then is then to have a comprehensive list of the things I shall need to change so that it becomes an independent machine on the network. I use them headless by the way and control usually through a PuTTy shell, but can connect a screen easily enough for setup purposes.
You could always boot the spare sd in the current pi and change the hostname and unset the static IP then put it in the new pi when you get it. Then it should be good to go and you can change whatever software you want on it and just put the other card back in the first pi.
The whole identity of a Pi is contained on the SD card. I used this procedure to add a Pi to my home LAN: I cloned the Raspbian install onto another card, and booted my first Pi from it. I changed hostname and also set the MAC address to an invented one by editing /boot/cmdline.txt, and in my ISP router's DHCP reservation settings (Virginmedia Superhub 2ac), assigned that MAC address to a static IP address. Then when I booted the new Pi from that card it was assigned the address I wanted. You can make your PI MAC address anything you like, e.g. mine are 00:00:00:00:00:10 and 00:00:00:00:00:11. I do everything headless; I never connect any of my Pis to monitors