Learned how to migrate a Rails app to accommodate changes to the database:

• ssh into the server
• Go to the root folder of the app (dhsi, in this case)
• run ruby script/generate migration what_you_want_to_do. This generates a framework file in the db/migrate folder. This will start with the migration version number.
• Edit that file to make the change you want to make to the db structure. (See Ruby docs for detail.)
• run rake db:migrate. When we did this, it successfully created the new field in the mySQL db, and modified the /db/schema.rb file.
• Restart the rails application (see instructions in ETCL docs).

However, although this seemed to go well, it didn't solve the problem; apparently it goes much deeper. ME is providing assistance at the moment; I've sent him some files, and I'm waiting for a response.