It depends.
A depletion MOSFET can be used as an ehancemnet MOSFET when it
is operated as an analog amplifier. However, a depletion MOSFET
can't replace an enhancement MOSFET when it is operated as a
digital switch.
When a depletion MOSFET is used as a digital switch, since the
junction between source terminal and substrate must be reverse
biased, the voltage of the source terminal of an N typde transistor
must be tied to Vdd, and it is completely opposite to an
enhancement MOSFET.
When a depletion MOSFET is used as an analog amplifer, the
source terminal and the substrate are both at the same potential,
just like an enhancement MOSFET.
