So went to my list -> List settings -> clicked on a specific existing field and there was no option to change the type of the field..
Is it not supported in SPO?
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Changing field types is supported, however not all fields can have their type changed. For instance on a task list, you can change the field type for Task Status but not Assigned To or Description. Assigned To is because it is a Person or Group field and Description because it is an Enhanced rich text multiple line of text field.
The reason for this is to prevent loss of data and/or because there is no common sense way of changing the content ie: when converting a Person field to a text field does the person making the change want to get the Display name, the user ID or one of the other ways of representing a user.