Transport between terminals is pretty quick via train. We landed in terminal A and had to get to Terminal E (International) and it took maybe 15 minutes...but....we were seated right near the exit door and the tram wasn't that far away...Delta will know your status and adjust so prob not a biggie issue re: missed flight (we had 55 minute layover)

Now cancellation is another issue....if your testing place is getting results in 36-48 hours maybe get tested a day later....not ideal but would cover you. There were over 100 empty seats on our ATL-SXM flight....and it went