Everyone on the itinerary can upgrade to EMS for free if there is a Mosaic on that same itinerary. Same goes for 2 checked bags and Mosiac boarding. The only perk that is limited to the Mosaic only is the free drinks, but my wife (non-mosaic) has occasionally been able to get to free drinks just by asking nicely after I've ordered something, and they see that I was Mosaic on the tablet they use for payment. I am Mosaic 1, but I imagine the upgrade at booking is the same for Mosaic 2 and above.

I've noticed that the upgrade does not have to happen after actually checking in. I've been able to move to EMS(price shows up as $0) at T-24 as Mosaic 1 just by 'managing flights'.