How soon before my flight can I change my flight?

Is two and a half hours enough time to change my flight before it takes off? Seriously, that's so close for comfort! I mean, technically it's the cutoff, but yikes. They're gonna slap me with a fee, aren't they? Ugh. Plus, the new flight itself might be way more expensive now, who knows? It's like, one time I tried to change a flight to visit my sister – last minute, I know, bad idea – and the price had practically doubled! It was insane. I ended up just keeping the original flight and rearranging my whole weekend. This whole flight change thing just stresses me out. So much easier to just, you know, book it right the first time. Lesson learned, right? Definitely makes me want to plan ahead more. Way less panic that way.