Is changing some letters in friends' names, such as writing "Mawsṭafa" instead of "Musṭafa" and "ʿAwmar" instead of "ʿUmar," considered a forbidden act?

Joking with friends by writing their names in a certain way or calling them directly by them is permissible, as long as it is not intended to mock or belittle them, for the elongation of the vowel is permissible in the Arabic language. The default ruling for joking is permissibility, as long as it does not lead to extravagance or involve anything unlawful. Indeed, the Prophet, peace and blessings be upon him, used to joke and would only speak the truth.