Best practice advice for updating emails in a lead nurture campaign

Hello

We have a 1-year-long lead nurture campaign. New people can enter the campaign throughout the year, but each calendar year we report on the results of the campaign as its performance for that year.

We want to update the content of the emails in the campaign for our 2019 cohort who will start entering the campaign from now on. 

Is it best practice to override the existing emails with the updates, and continue to use the same email over and over for different cohorts who enter the program each year, and therefore the report becomes cumulative totals of the overall performance of that email? Or, is it best practice to keep the existing email as a report snapshot for that year? And then create a new email with the new content, and replace the old email in the campaign canvas?