It's addressed in the T&Cs -- https://www.delta.com/us/en/skymiles...qualify/#terms
As long as you wait 60 days after the Headstart post to downgrade, it shouldn't be clawed back.

"Delta reserves the right to cancel, rescind, or void any MQDs or to cancel or downgrade Medallion Status (based on the adjusted number of MQDs earned) if the Card is cancelled or downgraded to a non-Eligible Card within 60 days after the date that MQDs are deposited or within 60 days after the Eligible Card renewal date. To receive the MQDs, your account must not be cancelled or past due at the time the MQDs are posted to your account. If Delta and/or American Express determine that you have engaged in abuse, misuse, or gaming in connection with this MQD Headstart in any way or that you intend to do so, Delta may not credit, Delta may freeze, or Delta may take away MQDs from your account or terminate your participation in the SkyMiles Program."