Upgrading to Debian Jessie can render your system unbootable.
A quick warning: When you upgrade your Debian installation to Jessie, you may find the grub2 can no longer install itself and you are left with an unbootable system. (Been there, done that, managed to save myself.)
Preconditions:
Your disks have been MBR partitioned so far in the past that there isn’t a megabyte hiding between the MBR and your first partition.
Your /boot is sitting on something which requires more than a tiny amount of code to access. In my case ext3 on an LVM partition.
When grub2 tries to update you will get messages about core.img being too big. At this point you may be screwed.
Am I safe?
If you are MBR partitioned, then check where your first partition is. If it starts at block 2048, you have plenty of space. If it is 63, you are in trouble.
If you are GUID partitioned, make sure you have a 1MB partition for boot loader.
How I recovered:
Since I was using LVM I plowed all the data off of my /dev/sda spindle using pvmove (many times), that involved de-mirroring things since I only had three spindles in the machine. Then I could repartition the drive. I went to GUID partitioning, where you explicitly make a boot loader partition for the use of grub2.
Then I could create an LVM physical volume on the remaining space of /dev/sda and shuffle all my data back into mirrors. The process was more complicated since I went through and reformatted each of the drives in case one blows and the BIOS chooses another, but all told it is possible to perform without a reboot.