I've been Hyatt Diamond for about a dozen years or so. I am also coming into Marriott as a SPG lifetime Platinum.

If Marriott credits my SPG nights and points towards lifetime Marriott, then yes, I'll be staying at Marriotts by default. If I have to start lifetime Marriott from scratch (actually, I have about 300 nights and half million MR points) then I'll just forget about status and stay where I stay, which is probably a good idea in the first place.