Funtamentally Air conditioners and Air Source Heat pumps are the same thing, aren't they? Are heat pumps any more efficent at cooling than an air conditioner? Can can you easily convert an air conditioner with a reversing valve to heat your house?

It can be done but easily is in the eye of the beholder. Nor would a right sized AC necessarily be a right sized ASHP.
Besides the reversing valve you would need a crankcase heater and temp monitoring controls among other things. Capacity would certainly be limited by the existing coils.
You could say they are fundamentally similar. I often point out to folks that there AC is already a heat pump which is why around here operating costs aren't that much more than a GSHP in the cooling season. One just has the capability to move heat in two directions and operate in cold weather.
j

I assume the crankcase heater would be to allow for defrost mode, correct?
 

Crankcase heaters keep the oil warm in the compressor ,
you would need defrost controll board ( defrost is ac mode with out the outdoor fan )
the corosponding temp sensors and freezstats
the reversing valve as you mentioned and the thermostat to controll it
a bypass for the metering device at the AH with a check valve
a metering device for the condenser
and the biggie an accumulator
Not to mention the aux heat strips and corosponding volts and wire size
most likely you would have to run new low voltage wires ( unless you have enough spare )
not an easy conversion but definitly possible

Incorrect.

Crankcase heater prevents liquid refrigerant from slugging compressor in cold weather.

Nuttin' to do with defrost mode.

The crankcase heater keeps the refrigerant from diluting the oil in the crankcase.  Diluted oil = poor lubrication.

Applications:

Crankcase heaters are used to overcome the problem of migration and condensation of refrigerant in the crankcases of compressors
used in air conditioning and heat pump systems. They are designed to keep the crankcase oil at a temperature higher than the coldest
part of the system to prevent migration.

Oh and by the way they keep a compressor motor from having to turn over with oil that is stiffer than molasses.
j

A heat pump may be slightly (on the order of 1 SEER point) less efficient at cooling than a straight cool unit owing to pressure losses within and thermal conduction through components of the reversing valve.

But yes, TG, air conditioners and heat pumps are essentially similar, except that a heat pump employs a reversing valve to allow the evaporator and condenser to swap roles.