I'm surprised you ask a question like that. xD
They say it's a 'method of evolution'. Not a hold item. Therefore, Knock Off probably won't work. But like Jaja said, it ain't revealed. So wait until October.
Hope I helped. xD
MrKijani editing this to make it correct :P
Mega Stones cannot be transferred to or from the correct holder: Thief and Covet do not take the Mega Stone, Knock Off does not remove the Mega Stone, Trick and Switcheroo fail, Fling and Bestow fail, and Symbiosis does not activate. Embargo and Magic Room also cannot prevent the use of Mega Stones to Mega Evolve Pokémon.
Therefore, no matter what you will not lose your Mega Stone