Can the BMD-340 be dropped into a BMD-300 footprint?
The BMD-340 is nearly compatible with the BMD-300. A new pin, VCCH, is outside of the BMD-300 footprint and must be connected on the BMD-340. This means the BMD-340 cannot be simply "dropped-into" an existing BMD-300 design. In short, you need to make couple minor changes to your board to accommodate the BMD-340. Note the BMD-340 is slightly larger than the BMD-300.
Is the BMD-340 footprint backwards compatible?
The BMD-340 footprint has been designed to allow for backwards compatibility with the BMD-300 and the BMD-301 modules. This allows the BMD-300 and BMD-301 to be placed onto a BMD-340 footprint (see the below footprint comparison). Pins 1 through 47 of the BMD-340 directly map the the same pins numbers on the BMD-300/301. This allows for both easy migration and enables tiered product design. If using a BMD-300 on the BMD-340 footprint, all of the new pads can be left disconnected, except for VCCH--which must be connected to VCC.
What are the differences?
Due to changes in the nRF52840 SoC used by the BMD-340, not all functions (such as SWO and TRACE signals) are found on the same pins as the BMD-300/301. Particularly of note is the reset pin function which on the BMD-340 is now available on P0.18 instead of P0.21 as with the BMD-300/301. To maintain pin for pin compatibility of the reset signal, P0.18 and P0.21 have swapped pad locations on the BMD-340 footprint. This is the same physical pin, only the logical pin assignment has changed. These differences with the BMD-300/301 are details in the table below.