consider Sysupgrade help for x86_64 - #14 by frollic
then use the online image builder to create a rootfs image containing the packages you need (the guide assumes you're using the standard one, but you can wget a customized image too).
copy the config from the live partition, to the partition of the new version (just mount the new one in the current one)
reboot into the new openwrt, once done.
if you have a fairly static setup, use root=/dev/sdX instead of root=PARTUUID=, in grub.
put it all in a script, and save it in /boot, since it never gets overwritten.
wget the image, zcat it onto the partition, and trigger the script.
you could just as easily run two squashfs partitions, instead of ext4, and they should
be easier to extend, if they're not resized on a live system.
just don't use the combined images, they will always reset your partition layout.