I recently went through the process of upgrading my raspberry Pis from Stretch to Buster. I had previously done this moving from Jessie.
Whilst I had no real issues with task following the guides that I found. I did discover one thing that concerned me, and was missing from the guides.
After upgrading to Buster, my /boot had less than 200kb of free storage.
I did some reading and found that Buster seems to need a lot more space in /boot than for previous releases. Buster itself now defaults to 256M of space for /boot
So I went about solving this issue.
I would suggest that you do these steps before upgrading just to avoid any possible problems.
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