[stylin19a]

great question.

That said, this article makes Flynn look naive in all this, almost virginal.
Fat chance. This aint his first rodeo. He's been at this stuff for a long time.

better question is: How does the FBI know he lied ?

Below was originally reported (From CNN) 02/15/2017. I couldn't find the FBI source for this.

"The FBI is saying that barring new information, they're not going to pursue charges against him. They add that in their conversations with him that he was forthcoming. He did not hide the truth. They say that if he forgot certain details of things he was not intentionally misleading. So if they don't learn anything new they're not going to pursue charges.”

What changed ?

Something else is going on.

[Garvin]

What changed ?
You answered your own question. Originally reported from CNN (Fake News).

[Hawthorn]

>> How does the FBI know he lied ? <<

Probably because our agencies try to monitor every conversation and communication by the Russian ambassador. It would seem to be nothing more than run-of-the-mill intelligence collection.