The reason "Chihiro" became "Sen" in Spirited Away

In Spirited away, Chihiro was taken her name and changed her name to Sen.

If you know Japanese Kanji, you can easily understand the reason. But if you haven't learnt Japanese kanji, it might be difficult to understand.

Source: Spirited away (Studio Ghibli)

the pronunciation of "千"

Source: Spirited away (Studio Ghibli)

In this way, her name was changed from Chihiro to Sen. Strictly speaking, she was deprived of her name.

Hayao Miyazaki was influenced by ゲド戦記のオリジナル

This idea: if you take away their name, you can control them, is based on "Earthsea" by Ursula Kroeber Le Guin. Hayao Miyazaki was influenced by her.

In the Book, it is said that

"then you will become able to learn its true name, and to understand the whole of its existence."

In the Earthsea saga, magic is searching for the true nature of things and by knowing their true names, to work upon the existence of the thing itself. Thus, the study of magic is nothing other than the study of reality itself.

In other words, A true name indicates a person's very existence. That is why, Yubaba deprived Chihiro of her name in order to work upon the existence like she did towards Haku.

The idea that names are important can also be seen in "That Time I Got Reincarnated as a Slime"

Source: マイクロマガジン社 'That Time I Got Reincarnated as a Slime 1'

https://www.amazon.co.uk/That-Reincarnated-Slime-light-novel-ebook/dp/B076H132D2

(*FYI, my dissertation topic was based on Mastercard's True Name campaign which is a innovative campaign to let transgender and nonbinary people to use thrier preferred name: True Name, on the credit card. In this way, True Name allow us to show our identity.)