How to set plugin auto-update Enabled by default?

I have the following problem. I have created a plugin for Wordpress. In the first versions of the plugin I forced users to auto update using this function:

function my_plugin_auto_update ( $update, $item ) {
$plugins = array ( 'my_plugin' );

if ( in_array( $item-slug, $plugins ) ) {
   // update plugin
   return true
} else {
 // use default settings
 return $update; 
}
}

add_filter( 'auto_update_plugin', 'my_plugin_auto_update ', 10, 2 );

The user had no way to disable auto-update.

Currently the plugin is already stable and I don't want to force users to auto-update anymore. When I remove the above function, the user can enable/disable auto-update from the Plugins tab. However by default this auto-update is disabled. How to make that after installing new version of my plugin auto-update is enabled by default but also user could disable it?