Is 32013 a drop-in replacement for 3651?
Dimensions are, respectively:


3651 - 0.8cm x 1.6cm x 1.1cm
32013 - 0.8cm x 1.6cm x 0.8cm


Distance between one axle and end of another also did not change. So for most builds it should work.

Incompatibilities include:


Obvious lack of studs
Use as in Lego Technic Universal Set 8034 as a locking mechanism for lift system, as seen on the box:


This answer was posted as Community Wiki, please add incompatibilities if you know them