  A planet's orbital orientation relative to an observer's line of sight
determines the chord length for a transiting planet, i.e., the projected
distance a transiting planet travels across the stellar disc. For a given
circular orbit, the chord length determines the transit duration. Changes in
the orbital inclination, the direction of the ascending node, or both, can
alter this chord length and thus result in transit duration variations (TDVs).
Variation of the full orbital inclination vector can even lead to de-transiting
or newly transiting planets for a system. We use Laplace-Lagrange secular
theory to estimate the fastest nodal eigenfrequencies for over 100 short-period
planetary systems. The highest eigenfrequency is an indicator of which systems
should show the strongest TDVs. We further explore five cases (TRAPPIST-1,
Kepler-11, K2-138, Kepler-445, and Kepler-334) using direct N-body simulations
to characterize possible TDVs and to explore whether de-transiting planets
could be possible for these systems. A range of initial conditions are
explored, with each realization being consistent with the observed transits. We
find that tens of percent of multiplanet systems have fast enough
eigenfrequencies to expect large TDVs on decade timescales. Among the directly
integrated cases, we find that de-transiting planets could occur on decade
timescales and TDVs of 10 minutes per decade should be common.
