The Interchangeability of Intention in Voluntary Prayers

Do voluntary prayers allow for interchangeability of intention?For example: If I intended to pray Witr with three rak'ahs but became lazy or forgot, and then I sat after the Fajr prayer until the sun rose, should I pray these two or four rak'ahs and intend them for both Witr and the sunrise prayer?

I say: They do not allow for interchangeability. The Sunnah of Duha is for Duha, the Sunnah of Witr is for Witr, and the Sunnah of the regular voluntary prayers is for the regular voluntary prayers.