Is it permissible to perform Ihram for Umrah from Cairo and wear sewn clothes over the Ihram, then remove them at the Miqat on the plane due to the cold weather?

Iḥram is not about wearing the Iḥram garments; rather, it is the intention to enter into the ritual. There is no harm in wearing the Iḥram garments at the airport. One can wear sewn clothes over them, and then, at the Miqat, remove the sewn clothes and make the intention for 'Umrah.