The 4.0 -> 3.6 FCV downgrade path in storage would acquiesce the system by closing/re-opening the WT connection. That was originally done to facilitate changing table logging settings.

4.2 development optimistically removed the close/open along with the table logging changes under the assumption that changing the file compatibility on reconfigure (which remains in the upgrade/downgrade path) did not need to be acquiesced to the same degree.