Yes, you can change a custom question's wording without creating a new survey. You can also change the wording for any selectable answers as well. *Note if you change your selectable answers after the question has responses, the responses will also reflect your new wording.
Macorva will not allow you to add more options to a question. For example, if you create a multiple choice question with three options to select from you are not allowed to add a fourth if the question has already been used in a survey. In that case, you would need to create new question.