I imagine they would leave it alone as much as possible, because they aren't sure what's bringing in all the traffic. Wouldn't want to be another Digg.
tl;dr: Digg was the original reddit - much bigger, reddit was the underdog.
Digg did a redesign that was absolutely horrible and created a huge backlash. Because Digg did not create a backup/proper versioning, they couldn't go back to the old design, and people migrated to reddit. The rest, as they say, is history.
