MacBook – Will changing the MAC address void the warranty or affect any existing programs

No, it will not void your warranty. If you refer to the top of these instructions, they clearly state only a deviation of the steps outlined may void your warranty:

Follow the instructions in this document carefully. Failure to follow these instructions could damage your equipment and void its warranty. Online instructions are available at http://www.apple.com/support/diy/

Typically, anything that is printed in the owner's manual is covered under warranty. RAM and changing the hard drive are both topics that are carefully covered, so legally speaking, that means they are user serviceable. Anything user serviceable is covered under warranty (following the above proviso of course).

Unless it explicitly states in the manual that such modifications must be performed by a qualified technician, you will not be in breach of your warranty.

Apple's policy has traditionally been that you can install RAM, drives and PCIe cards (on a Mac Pro), without voiding your warranty. However if there's evidence that you broke something during the install, they won't cover that, nor the 3rd party parts (naturally).