

By Vajiralongkorn.jpg: Amrufmderivative work: Sodacan (talk), CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=9094927
The royal succession in Thailand will be smooth when the King of Thailand dies, Palace observers say, but discussing the topic is highly taboo, due to the strict lèse-majesté laws in Thailand. Now that the health of their much loved King Bhumibol is quickly deteriorating there is now little choice but to discuss it.
King Bhumibol and his wife Queen Sirikit have three daughters and son, Princess Ubolratana Rajakanya (born 1951), Crown Prince Maha Vajiralongkorn (born 1952), Princess Maha Chakri Sirindhorn (born 1955), and Princess Chulabhorn Walailak (born 1957).
The 1924 Palace Law of Succession regulates the succession. The law essentially barres females, children of commoner wives or children of foreign wives from the throne and it re-affirmed agnatic primogeniture, or succession through the male-line by seniority. However, the constitution was amended in 1974 to allow the Privy Council to appoint a princess as successor to the throne. This applies only to the daughters of a King and may only come into effect following approval of the legislative assembly. This will only happen in the absence of a male heir.
The heir apparent to the throne is Crown Prince Maha Vajiralongkorn, who is the only son of King Bhumibol. In 1972 his father conferred the title “Somdech Phra Borama Orasadhiraj Chao Fah Maha Vajiralongkorn Sayam Makutrajakuman” on Prince Vajiralongkorn, designating him the Crown Prince and heir to the throne. If this title was ever revoked, it has not been publicly announced and so we may assume the title is still in place.

Princess Soamsavali Kitiyakara
By Ernst Vikne – , CC BY-SA 2.0
Despite the popularity of his father, the Crown Prince is not popular among the Thai people. The Crown Prince has been married three times. On 3 January 1977 the Crown Prince married Princess Soamsawali Kitiyakara, a first cousin on his mother’s side. They had one daughter, Princess Bajrakitiyabha, who was born in 1978. The Crown Prince began living with actress Yuvadhida Polpraserth in the late 1970s, but the couple did not divorce until 1993. He married Yuvadhida Polpraserth in February 1994 and by then they had already had four sons and a daughter. Just two years later he accused her of adultery and she and the children moved to the United Kingdom and later to the United States. They divorced that same year. The Crown Prince later abducted their daughter and raised her to the title of Princess. All their other children were stripped of their titles.

Princess Srirasmi By Vajiralongkorn.jpg: Amrufmderivative work: Sodacan (talk), CC BY 2.0
On 10 February 2001 the Crown Prince married again, to Srirasmi Suwadee. She was renamed Akharaphongpreecha. This marriage was not made public until 2005, when she gave birth to a son, who was named Prince Dipangkorn Rasmijoti. She was then raised to the title of Princess. However, in 2014 the Princess’ family was stripped of the royal named awarded to her, following allegations of corruptions against several of their relatives. The Princess was then also stripped of her royal titles and names and was officially divorced from the Crown Prince. She was exiled into retirement at an undisclosed location. So far, her son has not been stripped of his titles and he remains next in line after his father.
After the King’s death it is likely that there will be a long period of mourning, perhaps up to a year.
I am not sure if your opening sentence will be correct; lots of rumors about the succession.
Constitutionally and according to the royal family house law the crown prince is the new king; that being said Thailand has a long history of disregarding pesky constitutions; The CP is hated by many of the elite and regular Thais on the street; many feel he could bring down the monarchy.
The CP has cultivated the armed forces for years; most analysts feel that they are staunchly royalist; that being said leads to a BIG IF question–what if there is a move by his sister Princess Maha Chakri Sirindhorn (or people supporting her) to claim the throne. The princess has wide support among Thais of all walks of life and has cultivated the armed forces herself.
I have heard several scenarios; they all include the Privy Council, key officers in the armed forces and her mother Queen Sirikit who totally opposes her son taking the throne; in the event that her brother was derailed from taking the throne she would either become Queen Regnant or the Princess Regent for one of her brothers children. The fact that Princess Sirindhorn is 61 and has never married would sooth some traditionalist feathers about usurping the legitimate succession; she would be ‘married” to Thailand and having no children would allow her successor to returned to the proper line.
It’s all speculation; it could go very smooth or could be very bloody–I will pray for Thailand!