One of the benefits of the Historic Lectionary over the 3 year series is how catechetically tight it is. By that I mean that the readings are all very focused. If you follow the readings, preaching on the text, over the course of the year you WILL cover all major doctrines. The 3 year is more broad. The readings are a bit more open to picking and choosing a theme. You could accidentally miss an important point of doctrine one, or even several years in a row.
One of the drawbacks of the Historic Lectionary over the 3 year series is how very focused the readings are. Congregations are not doctrinal textbooks, where you just plug in the appropriate doctrine at the correct place on the page. Congregations are people living in the world. And it is a world of sin. That means that problems arise that must be addressed. The Historic Lectionary often does not leave much room for that to happen. So, what to do?
I joked about it a few weeks ago, but Luther gives a great example of how to use the lectionary if a specific problem arises. In his sermon for the sixteenth Sunday after Trinity (Widow of Nain’s Son), Luther makes the point that we are comforted by the Resurrection. And that, because of that, we show mercy when we spank children. Now, I don’t know what was going on in Wittenberg that week. But something was going on in Wittenberg that week. Because 0% of the theme of the day is devoted to discipline of children. Epiphany 1 is a good day to talk about discipline of children. But Luther must not have been able to wait for Epiphany 1. To say it feels shoehorned into the sermon is an understatement. And yet, he is correct on the doctrine. He says nothing false. He just does not really hew to the theme of the day.
So, pastors switching to the Historic Lectionary, follow the example of the blessed Reformer: Use the Lectionary as a guide and help to cover the entirety of Christian Doctrine throughout the year (as he recommends in the Large Catechism). But if you have a great need in the parish, use the Epistle, or Old Testament, or a parallel reading, or even just grab the pastoral shoehorn and speak God’s Word with reckless abandon.