%% decompose the appup
[{AppVersion,
	[{UpgradeFrom, UpgradeInstructions}],
	[{DowngradeTo, DowngradeInstructions}]}] = CONFIG,

%% and replace the version strings
[{"2.1.2",
	[{"2.1.1", UpgradeInstructions}],
	[{"2.1.1", DowngradeInstructions}]}].
