Well, technically... we have an example in modern Spain of an (almost) peaceful and willing transition without abdication:

• Franco was a dictator
• He appointed the King to follow in his steps
• Right after Franco died, the King did a 180 and facilitated a democratic constitutional referendum
• The majority, approved a democratic constitution, leaving the Executive power split in two: the King remains the leader of the military (in time of war, and mostly in name otherwise), while an elected President is the leader of the rest.

Other than a failed coup attempt by a faction of the military who wanted to go back to the previous system, it was a reasonably peaceful transition from full dictatorship, to a "parliamentary monarchy".

It can be done, if people are willing.

(PS: an abdication came much later, because of some not fully transparent money deals and tax evasion schemes, leaving his son as the new King)