BOGO - Add a line using older phone

Looking to get two new iPhone 12, and to do so we need to add a new line.   We’d like to use the new line for our grandparent - do we have to use one of the new phones on the new line or can we use the new phones on existing eligible lines, and use an old phone on the new line?   Our grandparent doesn’t need anything fancy,  and figured it’s a win-win for everyone.    Not looking to cancel any lines - just add one line with an older phone and use the new phones (BOGO) on existing. 

thank you!