There are numerous ways to post-process. After you got your data in au as
data=EntityValue[EntityClass["Planet",All],{"Name","Image","DistanceFromEarth"}];
You can use any of these methods and there probably many more:
data/.x_Quantity->UnitConvert[x,"LightMinutes"]
MapAt[UnitConvert[#,"LightMinutes"]&,#,3]&/@data
Transpose[MapAt[UnitConvert[#,"LightMinutes"]&,Transpose[data],3]]
Cases[data,{a_,b_,c_}:>{a,b,UnitConvert[c,"LightMinutes"]}]
