Code's not exactly a house remodeling project. You don't have to take a hammer to your old bathroom before you can make the new one. You can keep both buildings around as long as you want, and if you find you forgot to put a kitchen in the replacement, you can always route your users, within reason, back to the kitchen that's sitting around in the old house.

The lesson? Never raze the old house until you're completely sure the new one is working. There's no great reason to destroy code that you're replacing. Cut off the calls to the code, but unless there's a really good reason, keep it around until you're done with your testing and roll out. You'll thank me later.