You can move the SD card onto a pc, zip up the files and push them to a second SD card. I did that when I first set up Weewx on a Raspberry Pi. but I wouldn't do it now. So much stuff changes so often that I now backup the conf files, skin files, and other custom scripts using tar and store them in github. The ones that I mess with frequently I have as separate files. For the database I have a script that runs nightly and copies it to google docs/drive.
If (inevitably when) I rebuild the system, I'll reinstall the up to date OS and Weewx version and then overlay my customized files.