The Pau Cin Hau logographic script was reformed in 1931 AD, moving from a logographic language to an alphabet. While it is not unusual for a phonetic writing system to evolve from a logographic writing system, it is very rare to do so in only 29 years.
Update: I have found several other examples of a script starting logographic and then shifting to something else. Sequoyah experimented with logograms before shifting to a syllabary for Cherokee; same with King Njoya for Bamun (in fourteen years) and Uyaquq for Yugtun script.
In addition to consonants and vowels, the script also has 9 characters for final consonants and 20 tone marks.
The term “tone mark” is misleading, as they actually represent much more than the pitch. They also represent vowel length, the glottal stop, and punctuation.
The American Baptist missionary Joseph H. Cope developed an orthography (spelling rules) for Pau Cin Hau using the Latin alphabet, which ended up displacing the Pin Cin Hau alphabet. The use of the Pau Cin Hau alphabet is very limited now.
Links: Unicode proposal