I will have to investigate this inplace upgrade.
When I booted from a recovery USB, it wanted to format my drive and re-install.
I did and restored my latest backup--- all my programs and personal stuff was gone!
I had an image of the disk from an image program and put everything back but I would have been totally screwed without the image.
BTW, I do not store any data files on the OS drive.
Yes, I never store either priv data on OS partition. But the fact is, we cannot update the sys. 8(