Before each upgrade, check to see if the left file has changed. If it has, recreate the plugin based on the new code and the changes from the commit history for the file on the right.
backend/app/model/ead_exporter.rb------------------>plugins/local/backend/model/ead_exporter.rb
frontend/app/views/site/_branding.html.erb--------->plugins/local/frontend/views/site/_branding.html.erb
public/app/views/shared/_footer.html.erb----------->plugins/local/public/views/shared/_footer.html.erb
public/app/views/shared/_header.html.erb----------->plugins/local/public/views/shared/_header.html.erb
public/app/views/shared/_navigation.html.erb------->plugins/local/public/views/shared/_navigation.html.erb
public/app/views/shared/_record_innards.html.erb--->plugins/local/public/views/shared/_record_innards.html.erb
public/app/views/shared/_request_form.html.erb----->plugins/local/public/views/shared/_request_form.html.erb
The following files only have certain values overridden and cannot break code, unless there is a fundamental change in variable naming.
public/config/locales/en.yml----------------------->plugins/local/public/locales/en.yml
plugins/local/public/views/shared/server.test.css
plugins/local/public/views/shared/_takedown.erb
plugins/local/public/views/layout_head.html.erb (this no longer seems to be in the core codebase, but we continue to call it)
plugins/local/public/views/site/_branding.html.erb
plugins/local/public/views/site/_header.html.erb
plugins/local/public/views/site/index.html.erb
plugins/local/public/plugin_init.rb