I am going to perform ‘Umrah, and I wear the Niqab. When should I remove my Niqab, right before the Meeqat (place where one should assume the state of ihram) or after getting ready for ihram (state of ritual consecration) in my country?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

A pilgrim is not required to assume ihram before reaching the Meeqat, by land or air. However, if he assumes ihram in his country, then that is valid, but he will thus be required to avoid the things forbidden for a person in ihram once he assumes it. The Niqab is one of those things, as related in a Hadeeth in which the Prophet, sallallaahu ‘alayhi wa sallam, said: "A woman in ihram should not wear a Niqab or gloves." [Al-Bukhari]

In Al-Moghni, Ibn Qudaamah said:

He is recommended to not assume ihram before the Meeqat; but if he assumes it, it is a valid ihram. There is no difference of opinion regarding the fact that the one who assumes ihram before the Meeqat becomes in a valid state of ihram, being subject to the rules of ihram. Ibn Al-Munthir said, "The scholars unanimously agreed that the one who assumes ihram before the Meeqat becomes in a valid state of ihram."

Accordingly, if the questioner assumes ihram in her country, she is required to take off the Niqab. If she assumes ihram at the Meeqat, however, then she may wear the Niqab during her journey until she reaches the Meeqat, whether she prepares for ihram in her country or aboard the plane before reaching the Meeqat. Indeed, the core of ihram is the intention. Whenever the intention for ihram is made, its restrictions should be avoided. 

Allaah Knows best.