1. Don’t change it. Baidu the following keywords "name rare characters", how many parents and children are suffering and regretful because of the uncommon characters!
2. A name is a symbol. On the one hand, it expresses the expectations of your elders or yourself, and on the other hand, it is convenient for others to call you. When using dialect or Mandarin to address homophones with some vulgar words, do not modify them as much as possible.
3. I guess you are a girl, and the name "Sheng Nan" is very good. "Shengnan and Shengnan" are both incomprehensible names. They are both difficult to recognize (both characters "Shengnan" are polyphonic) and not pleasant to hear ("Shengnan" and "shènɡnàn" both have the fourth tone)