> I don't see how it's "easier" to change an actual conversation than it is to completely fabricate one.

It's easier because, when you fabricate an entire conversation rather than one sentence, you have to make up more things.

That's the purely technical argument. There are also psychological considerations on the part of the officer, people are more likely to rationalize making up one little thing than making up an entire conversation.

Yes, they can ask you all kinds of questions about what you said and make you appear quite talkative.

Say nothing and you can truthfully say repeatedly that you said nothing. You'll be able to consistently tell the truth on the stand without having to explain any of your statements, statements that the prosecution will attempt to twist into incriminating statements.

Of course, if you do the same to the prosecutor before trial, the chances of you making it to trial go down. Prosecutors like to win their cases.

Only in the very technical, abstract sense. Interrogations, testimony and memories are about psychology, what has actually happened is only superficially related. When you have a long conversation with someone, it's easy for them to remember things you didn't say because they think they heard them. Watch the video linked above, for the example about the story the guy read to the audience. This is exactly what happens to everybody - I myself thought I was smarter than that, but I'm really stupid and stubborn like that and it took me at least 10 times being proven wrong before I internalized that no, the things I think I see and hear and remember are quite likely to be different from what actually happened (in criminal psychology and evidence classes, this was).

Anyway yes it's easier to change a conversation than to fabricate one; because "changing" can mean many things - intentionally lying is the least common occurrence, simple "neural" errors happen a lot more often.

You don't know ahead of time what's going to happen. That's the key here.

Maybe they'll lie (probably not that common), or maybe what you said will somehow implicate you in a crime, or maybe...? That's why it's important. It's not after the fact knowing the police lied but beforehand when you don't know how things will play out that remaining silent is important.