i upgraded from stretch to buster a couple months back and ran into a few problems. This is roughly how I did it, I can post the Linux commands later tonight when I’m on my computer:
1. Ran the update scripts published on the web
2. Created two partitions on a new SD card so I could have a bigger BOOT partition. It is a LOT easier to crest it at 500 meg from scratch than to try to resize a boot partition
3 copied the contents of the boot partition in old card to new card
4. Mirrored the second partition over to my new card and then Resized it using gparted on another Linux box
i had to fix a couple little things in the boot partition like the partition reference of my new card.
I had to reinstall mumble with a new version but it worked perfectly and all rigpi software was fine.
I also moved my second partition to an external SSD and this rpi4 is FAST.