If you already have upgraded LINE to the incompatible version (8.12+) there is a trick to downgrade LINE without uninstalling the app and losing all your messages.
Clone the LINE 8.11 APK (see https://goo.gl/36QXQc) setting the Version code under Developer options to the version code of the newer, currently installed LINE app. For example, for LINE 8.14.2 this is 19081402. Basically you can use any number larger than the installed app’s version code.
This allows you to replace the cloned app by the older, compatible version.