I attended Low Mass at an SSPX chapel yesterday. There were no Leonine Prayers--instead we were to recite some prayer to St. Joseph.
What is this about? We were confused.
Ask the pastor why the change. We are traditionalists, we don't change things at our whim.
All change except away from evil, is the most dangerous of all things. (Plato)
If he wants to do the St. Joseph prayer, do it before mass with the rosary, or do it after the Leonine prayers.
Beware, he may be setting you up for eliminating the prayers after mass all together.