What i did (& tend to always do) is buy a new cheap HD (or move to SSD?), make a 100% duplicate of your old drive, then let the upgrade happen on 1 of the 2 drives.

Worse case, you just swap back the old drive & life returns to normal. Plus if something breaks, you can always mount the old drive as "D" or whatever & access all files. That is also how i rebuild my drive "C" once a year, except I do a fresh install on the new/other/reformatted drive, saving the old working drive as backup. Worstcase, put the old drive back in as C and I'm back to where I was last week.