Unfortunately, there is no automated method for converting multiple SimpleViewer galleries to Juicebox galleries in bulk.
You can swap each gallery's 'svcore' folder for a jbcore' folder (which shouldn't take too long) and Juicebox can read a SimpleViewer gallery's 'gallery.xml' file so you wouldn't need to alter the contents of the configuration file (although you'd need to rename it to 'config.xml' or add a configUrl to the Juicebox embedding code) but actually changing the embedding code in your web pages is likely to take up most of the time in the conversion process.
Depending on how your website is structured (and whether or not you use a baseUrl for your galleries), swapping the embedding code over for all your galleries could be a relatively easy process of copying and pasting (but, admittedly, if you have many galleries, it could be quite time consuming).
Also, please bear in mind that although Juicebox can read a SimpleViewer gallery's 'gallery.xml' file, Juicebox was written from the ground up (and is not just a HTML 5 translation of SimpleViewer). As such, Juicebox has a different feature set to SimpleViewer and not all SimpleViewer configuration options have direct equivalents in Juicebox. With this in mind, you might like to use JuiceboxBuilder to convert your SimpleViewer galleries (as documented in the Converting a SimpleViewer Gallery support section) as you could tweak your galleries via the JuiceboxBuilder interface during the conversion process.