Is it possible to rename an index using a rails migration?

You can execute arbitrary SQL in your migrations as well.

We have some helper methods that add foreign keys to our tables:

def add_foreign_key(from_table, from_column, to_table)
  constraint_name = "fk_#{from_table}_#{from_column}"

  execute %{alter table #{from_table}
            add constraint #{constraint_name}
            foreign key (#{from_column})
            references #{to_table}(id)
           }
  end

You can use any SQL your database supports.