One explanation of how Swami Yogananda became Paramhansa Yogananda

"If examined, the speech Yoganandaji gave at that reunion celebration reveals much. The first noticeable thing is that up to that point, he had never used the name “Paramhansa Yogananda”; he was always known previously as “Swami Yogananda.” It was in his autobiography that he first makes it known that Gurudev Swami Sriyukteshvarji bestowed the title of “paramhansa” on Yoganandaji while he was in India. However, the writer and others who were close to him were not aware of this happening. Swamiji himself had never mentioned this news to anyone. Any sannyasi [renunciate], sadhu [sage] or brahmachari [celibate aspirant] can be given the title of “paramhansa”; this is one of the wonderful aspects of Hindu culture. Swami Satyanandaji had said, “Yogananda was attracted to the ‘paramhansa’ title since childhood.” One humorous incident comes to mind regarding this. Once at dusk, Swamiji was urinating at a drainage spot in the front alley by Gurudev’s house in Serampore. Gurudev was standing on the second floor veranda and speaking to someone when he jokingly remarked, “Yogananda has become a ‘paramhansa’ [great swan]!” Ananda-da [Ananda Mohan Lahiri] and the writer were standing by the front door of the house. They and Swamiji heard Gurudev’s ironic joke. Swamiji came to the front door and said, “Ananda-da! Did you hear? Swamiji [Sriyukteshvarji] called me a ‘paramhansa’ [great soul]!” Ananda-da softly said to the writer, “You’ll see! Yogananda will use this title in the future, for certain.” It is not possible to understand why Swamiji did not announce at the speech of New Year’s Day in 1937 that Gurudev bestowed this title on him, and why he revealed this only in his autobiography later." 

Dasgupta, Sri Sailendra Bejoy. Paramhansa Swami Yogananda: Life-portrait and Reminiscences (Kindle Locations 2381-2385). Yoga Niketan, Inc.. Kindle Edition.