How did it happen **contains spoilers**
The knock on the door from Elder Kennedy caused Mr. Reed to leave the girls and answer it. When Reed left the girls ran up the stairs and tried to get the box of matches. It was at that moment the magic trick happened and the body was swapped, but how?
The body was discarded under the floorboard under the table. Regardless of how physically weak the enslaved women were, how was it possible to move the table, open the trap door, dump the body, close the door and put the table back without making noise? When we saw Sister Paxton open the hidden door and go down there for the first time she had to dig through the mud to find the handle to open the hatch door. If it had just been used the handle and/or footprints would have been visible.
Also, the bike lock. It was used to lock the enslaved from leaving, but they were in cages and had been down there for months/years so what was the purpose of locking the door?