Originally Posted by Anarion
What about whenever a program tried to do anything with an older, existing program that had the same number defined as a different variable? Or if people went and grabbed existing code that already was using pi and tried to change or expand it, they'd have to go through and replace everything with tau, right?
There might be some special cases, I can not rule that out. Mostly when you use someone else's code or interact with an existing program, it is through an interface of some sort. How their code works exactly shouldn't matter to you. If it uses pi somewhere, that's not going to affect you, you trust that their implementation is correct and it does what it advertises (of course you might want to check if you don't trust them, but that's another matter).
What you need to be aware of is what input it requires and what its output is expected to be. If it requires pi as input, then obviously you need to give it pi (or half tau), and if it spits out pi then obviously you shouldn't treat that as tau.
Also, I want to say that there's nothing wrong in using the value 3.14 (plus however many decimals you need) if that's what is most natural in a practical application. If you understand that 3.14 is half tau, and you happen to need half tau, then surely you write 3.14 not 6.28/2.
Heck, you can even call that pi if you like, as long as you understand tau and use that as the base definition. Start with tau, learn the math using tau, then do whatever you wish once you understand it.