Derive Reference Potential Evapotranspiration from Potential Evapotranspiration
I agree that terminology about evapotranspiration is confusing especially when using the expression "potential evapotranspiration", as stated in the FAO-56 paper:


  The use of other denominations such as potential ET is strongly discouraged due to ambiguities in their definitions


Definition



So let's refer to the FAO terminology:


Reference evapotranspiration ($ET_0$) refers to the evapotranspiration observed on well watered grass  for given meteorological conditions (radiation, temperature, humidity, and wind speed).
Crop evapotranspiration under standard conditions ($ET_c$) refers to the evapotranspiration observed on well watered specific crop for given meteorological conditions. The relationship between $ET_c$ and $ET_0$ is given by:


$$
ET_c = K_C\times ET_0 \tag{1}
$$


Crop evapotranspiration under non-standard conditions ($ET_{c,adj}$) refers to the evapotranspiration observed on a specific crop under water and environmental stress for given meteorological conditions. The relationship to $ET_0$ is either defined using a stress coefficient $K_s$:
$$
ET_{c,adj} = K_s \times K_c \times ET_0 \tag{2}
$$
Or using an adjusted crop coefficient $K_{c,adj}$:
$$
ET_{c,adj} = K_{c,adj} \times ET_0 \tag{3}
$$


Confusions



Now, you may find different common usage for the terms potential or actual evapotranspiration that leads to your confusion. 

Potential evapotranspiration is sometimes referred to $ET_0$ or $ET_c$ or even less strict definition such as atmospheric demand for water. 

Actual evapotranspiration  is referred either to $ET_c$, $ET_{c,adj}$ or less strict definition such as the amount of water evaporated.

Pragmatism



Regarding your issue, pick one:


Read to find out what are the definitions involved in WEAP and HAR
Look at the data and find out if potential evapotranspiration refers to grass (definition 1) or to specific crop with a given coefficient (definition 2, eq.1)
Use the other HAR data (everything is available) to compute $ET_0$ using FAO-56 method.
Last but not least, do not bother that much as evapotranspiration is full of uncertainties and you could not proove which of $ET_0$, $ET_c$ or $ET_{c,adj}$ best represent your actual evapotranspiration. You are interested in the relative dynamics of $ET$, choose realistic ranges of landcover coefficient and calibration will do the mass balance.