I dual boot Win 8.1 and opensuse on my main desktop, Win 7 and opensuse on my laptop.
I’m not particularly concerned with problems. However, I am holding off the Win10 upgrade until I hear more reaction from users.
Where is grub installed? (In the MBR or in a partition?)
For my Win7 system, grub is installed in a partition (actually “/boot”). My expectation is that if I upgrade, I will then have to change the active partition so that I can get back into opensuse.
If, however, you have grub installed in the MBR, then Windows can cause greater problems. If I were in that situation, I would probably do:
# dd if=/dev/sda of=backup_MBR count=1
to save the MBR content into a file. I would save to an easy to find location (perhaps even a USB).
Then, after the upgrade, I would boot linux live media and try:
# dd if=backup_MBR bs=440 of=/dev/sda count=1
to restore the MBR content. I don’t know if that would actually work. Grub uses the unassigned space between the MBR and the first partition. I don’t think Windows overwrites that, but if it does then a grub reinstall would be needed.
There’s another possible problem. Windows might add partitions and change the partition numbering. In that case, it is necessary to also edit “/etc/fstab” (or, maybe not if it is using UUID for mounting).
If your box uses UEFI, then the problems are different.