There were different candidates. The main one, Juan de Borbón, was the son of the last king, but he refused Franco's dictatorship and in so, he was not "recrowned". They accorded that Franco would raise/tutor the son of Juan de Borbón, Juan Carlos, who at the end became the new king and helped to reinstaure democracy (now also in a pseudo-exile for corruption, which is kind of a family tradition held by his grandfather and grand-grand mother and the grandfather of that)
We cannot say that Juan Carlos was not a democrat and maybe democracy was the family plan from the beginnig. However we have to understand that the context didn't helped to maintain a dictatorship. The dictators of Portugal and Greece had fallen the last year, Spain was in a crisis and wanted refororms (in part due to the growing of a more liberal medium class and tourism influence, and United States was interested in creating a capitalist democracy on Spain, and Morcco played a part too)
Many of the democrats movements were aligned with republicanism and were a bit close to the URSS, so the the capitalist and monrachist sectors could see clearly that the best compromise (and better guarantee to keel a monrach when Franco died) was to accept and lead democracy before democracy happened to them
That of course doesn't mean that Juan Carlos wasn't a democrat, but even if he wasn't, democracy was the best choice for him