Just add new ones overtop.
It works but the duration of these transitions won't change,
if I change the default duration during two "apply default transitions to selection" actions.
If you go to Edit>Preferences>General and change the Duration, BEFORE you Add Default Transition, does this get you the new Duration?
I get the right duration when the first time "Add Default Transitions to selection".
However, when I modify the duration settings and "Add Default Transitions to selection" again, the transitions did change if I change the default transition from A to B, but the duration is still old.