In order to upgrade your galleries so that they support mobile devices, your galleries need to use the latest version of SimpleViewer.
Prior to supporting mobile devices, SimpleViewer was essentially just a single SWF file which was embedded directly into a web page using SWFObject.
Therefore, as your galleries were built dynamically by a PHP script using an outdated version of SimpleViewer, you would need to modify the PHP script to support the current version of SimpleViewer with the new files and embedding code and rebuild your galleries.
Using WP-SimpleViewer is essentially just a quick and easy way to recreate your galleries without having to modify your PHP script. Trying to upgrade your old galleries manually would be an extremely time consuming task (and perhaps difficult to do within a WordPress environment where content is traditionally generated dynamically).
There should be no need to update your NextGEN Gallery plugin and no need to re-upload any of your images. You would be creating a new gallery (to replace an old one) but this is basically what you would be doing by upgrading a gallery manually anyway. The only things that stays the same are the images.
WP-SimpleViewer uses the latest SimpleViewer files and generates the required embedding code to be compatible with mobile devices.
Using WP-SimpleViewer is by far the easiest way to create a SimpleViewer gallery sourced by NextGEN Gallery images within WordPress.
Just delete an existing gallery from a page or post and create a new one (sourced by the same NextGEN Gallery images) in its place. Once WP-SimpleViewer has been installed, you should be able to 'upgrade' a gallery in this manner in less than 30 seconds.
SimpleViewer Support Team