Extreme smart move for Tesla, it's a win win situation.
Tesla needs to increase production, that includes parts and labor to prepare for M3, they're making 90k MS/X in a year now, but Tesla can't suddenly flip a switch and start making 500k in a year. Tesla needs to sell more cars today to position them to hire more labor and parts so they can be ready when M3 production starts.
Why a win for M3 reservation holders? Anyone buys a MS now will probably cancel the reservation for M3, bump everyone else up.