I am preparing to do a Drupal 7 to Drupal 8 migration. This is not a pure like for like migration. Whilst many of the content types are similar there will be some differences between the Drupal 7 and Drupal 8 content types and paragraphs (i.e. site architecture improvements).
I would have thought that the easiest way to do the migration would be to completely rebuild the site in Drupal 8 and configure and customise it the way I want and then just migrate the content (e.g. users, taxonomy, nodes, etc.); however, according to the Drupal documentation one should not configure the site. From the Drupal "Preparing a site for upgrade to Drupal 8" page
Do not configure the Drupal 8 site
Keep in mind that the upgrade process will overwrite configuration on the Drupal 8 site, so do not do any configuration of the Drupal 8 site until after the upgrade process is complete. This means you do not have to create all the content types and fields manually before running this upgrade. The Migrate module will create them as a part of the migration process. In fact, it is a common best practice to install Drupal 8 using the Minimal install profile, because the Standard profile introduces its own configuration that you might not want to use.
Couldn't I just build and configure the Drupal 8 site just the way I like it and then just migrate the content being sure to customise the migration in order to adjust for any architectural changes? This idea of migrating all content and configuration and then making content changes afterwards doesn't make sense to me.