Yes, DCN covers it all.

Sorry, I can't help you with how to do it in Lacerte (I use ProSeries).

Have you compared the results with adding it to 2019, versus amending 2018?  You may want to do that.

As far as Form 3115 goes, it might be easier to amend 2018 than to file Form 3115 with 2019.  But comparing the tax due in both scenarios would be a good idea.