I had this happen to me last year on a flight to from DTW to CDG, but Delta sent me an email informing me of the change:

"We wanted to let you know that unfortunately your seat assignment on
Flight DL98 on March 25 has been changed due to circumstances beyond our
control. We've done our best to give you a similar seat; your new seat
is 8A.

We know you have seat preferences and are really sorry we had to change
your seat. You're welcome to review your new seat assignment on
delta.com or the Fly Delta app where you can view your seat and select
from other available seat options.

Again, we apologize we weren't able to honor your original seat choice
and for any inconvenience this may cause. We're grateful for your
SkyMiles loyalty and look forward to seeing you on board again soon."