Unfortunately, it's not as simple as flipping a switch to send content to the old version of the app. The problem is that new content is in a completely different format – coded differently – and converting it to the old format is a significant amount of work. The current content format, HTML 5, has many benefits that the old content formats didn't have – for example, highlighting any part of a manual (included bulleted lists) and easy cross-platform publishing.
I don't know if there are plans to convert the Howard W. Hunter manual to the older format, but I doubt that there will be resources to support the older format much longer. The current solution to this problem, for those who aren't able to use the newest version of the app, is to use the manual at LDS.org in a web browser; or, to download a PDF of the browser and use a PDF reader app to study the manual offline.
Samuel Bradshaw • If you desire to serve God, you are called to the work.