Now it is possible to lock bookings in units (meaning they can't be moved from a unit) when doing Check-in. 

This is a setting that needs to be enabled for each account separately: 

Why is it useful?

Cleaning is often not finished when Check-in starts. Therefore we need to have the flexibility to move bookings to clean units when guests arrive.
This lock function provides a hotel a safety net that eliminates a risk of moving a booking that's already checked in (which can cause a guest going into a room that's already occupied).

The Room and Unit field located on the Summary tab will be locked for bookings. When using the drag and drop feature the booking will simply not move into a different unit.