I've been travelling through South America for some time, passing also through Peru, Ecuador and Colombia.
In Peru and Ecuador I've eaten Lucuma several times without knowing anything about the two types ("lowland" and "highland"). I've had good ones and bad ones. The fruit that was most similar in taste to me was the macauba or bocaiuva palm fruit (Acrocomia aculeata) while being totally different in texture.
When arriving at the hot and moist tropical coast of Colombia I got to know the "Zapote costeño", the Mamey Sapote. There I heard that people graft Lucuma scions to Mamey Sapote rootstock to adapt it to the hot and moist climate. Does anyone have experience with that?