The 'skinny' from Mario B. of DMV is :

After the conversion, have the Bureau of Automotive Repair inspect the
vehicle at a Referee stainon.  If in order (read meets diesel requirements
and is NOT a commercial license) the referee will issue a certificate of
exemption and mark the database to indicate the vehicle is exempt from smog
requirements.

After the vehicle has been exempted by the BAR, DMV can review evidence of
the change including the exemption certificate and change the mode of power
from "G" to "D".

BAR authority is California Health and Safety Code Section 44011.

NB: not sure if the issues on engine exchange requirements are called into
play here.  REF: motor must be from a same or later year...and so on.

More to come on this

Al