As (almost) always, the expert's answer is: "It depends". How risky is the change, how big the consequences, how un-nice is the code before, how easy is it to test that the code still works afterwards, etc...
FWIW, I tend to err on the side of "do it", and I usually do it. But I have been in a situation where a customer asked for the risk level, I answered to the best of my knowledge (quite low but it's hard to be 100% sure), and they declined the change. The consequences of a bug would have been pretty horrible, too. Hundreds of thousands of (things) shipped with buggy software that is somewhat cumbersome to update.