I read that if you book a ticket for a toddler who is < 2 at the start of the trip, but > 2 on the return, they can travel as a lap infant both ways.

Is this correct? If so, I am hoping to book a flight as a lap infant using VS miles. I can find availability for the departure, but not the return. Would I be able to book the departure using VS miles, and pay cash for the return and have that be considered the same itinerary (and lap infant both ways)?