Devavratha became Bhishma after he took bhishana pratigya ('terrible oath') — vow of life-long celibacy
https://www.google.com/search?q=jaisa+ann+waisa+mann+mahabharata
https://www.google.com/search?q=Bhishmacharya
https://www.google.com/search?q=Bhishma+Garga
https://www.google.com/search?q=Matsyagandha