What is the reason for the change in wording from "al-Mawla" to "al-Wali" in the mentioned verses, and what is the significance of each?

"Mawlana" and "Waliyuna" in Quranic usage carry the same meaning: that Allah is the helper and supporter of the believers. Linguistically, they both denote closeness. There is no difference between them in Quranic contexts; rather, they are a stylistic variation in expression.