  We present the first statistical analysis of complexity changes affecting the
magnetic structure of interplanetary coronal mass ejections (ICMEs), with the
aim of answering the questions: How frequently do ICMEs undergo magnetic
complexity changes during propagation? What are the causes of such changes? Do
the in situ properties of ICMEs differ depending on whether they exhibit
complexity changes? We consider multi-spacecraft observations of 31 ICMEs by
MESSENGER, Venus Express, ACE, and STEREO between 2008 and 2014 while radially
aligned. By analyzing their magnetic properties at the inner and outer
spacecraft, we identify complexity changes which manifest as fundamental
alterations or significant re-orientations of the ICME. Plasma and suprathermal
electron data at 1 au, and simulations of the solar wind enable us to
reconstruct the propagation scenario for each event, and to identify critical
factors controlling their evolution. Results show that ~65% of ICMEs change
their complexity between Mercury and 1 au and that interaction with multiple
large-scale solar wind structures is the driver of these changes. Furthermore,
71% of ICMEs observed at large radial (>0.4 au) but small longitudinal (<15
degrees) separations exhibit complexity changes, indicating that propagation
over large distances strongly affects ICMEs. Results also suggest ICMEs may be
magnetically coherent over angular scales of at least 15 degrees, supporting
earlier theoretical and observational estimates. This work presents statistical
evidence that magnetic complexity changes are consequences of ICME interactions
with large-scale solar wind structures, rather than intrinsic to ICME
evolution, and that such changes are only partly identifiable from in situ
measurements at 1 au.
