Chris
Here is my "UN-scientific" and not proven advice (follow with care);
1) Backup SD card - as it seems you already done, or just get a brand new SD card (to be extra safe and troublesome)
2) Copy Weewx directories/folders, to PC or other - formerly Weewx directories (may vary):
- /etc/weewx
- /usr/share/weewx
- /var/lib/weewx
3) Upgrade card, and copy Weewx directories/folders back on.
If all goes well, it should work.
Xant