Adding to this post to make it absolutely clear that you _should not_ be using the 3.13 release at all, really.
To do so it to cast off literally years of bug fixes, improvements, features and optimizations. There really is a very highly limited subset of devices that I would actually suggest people prefer the release over the development builds, and that list doesn't include any of the SanDisk devices.
I would very highly advise updating. You can do so without reinstalling the bootloader, and without data loss. Honestly, you don't even need Rockbox Utility to manage this for you at all. That is only (arguably) useful for initial installs. You may simply download the current development build for your device and extract the archive to the root of the device, while allowing it to replace any and all original files.
This is all Rockbox Utility is doing for you anyway. I guess it seems easy to think that there is more to it than this, but behind the scenes all it is doing during updates is providing a graphical front end for downloading and extracting a .zip archive. There's no fancy magic involved here for updating, all that magic revolves around patching the original firmware with the bootloader, which you have already done and will never need to do again unless instructed to.