I found it a bit tedious to use the JuiceboxBuilder to move multiple galleries from sv2 to jb and therefore have built a PHP-CLI-tool which does the heavy lifting of moving the configuration from gallery.xml to a config.xml for me:

https://github.com/mahok/jb-tools

Rule of thumb: If you don't know what github is, you won't need this tool.

This tool is aimed at web developers/designers who know how to work with PHP from the command line!
Right now all it does, is take a gallery.xml and transfer most of the configuration into a config.xml, taking into account some of the renamed attributes (e.g. title > galleryTitle), but not stuff like backgroundTransparency which requires backgroundColor to be converted into a rgba-value. On top of that, currently it is pretty much exactly what I needed for my galleries, which use external jbcore/ and gallery/-folders.

In other words, it very likely requires additional work on your end and for most of you is not what you are looking for. In other words, use at your own risk.

For an explanation on what the script does and how to use it, see the Wiki-page: https://github.com/mahok/jb-tools/wiki