ah windows.old
yeah if you dont format the HD when presented with the option youre stuck with a parallel install, this is what im doing, cause space is not a premium for me. not sure when/if Windows 10 does present a format HD option, this is what im asking for specifics on too. i wouldnt worry too much about it. i have only seen windows.old create issues with MS Outlook .pst files / profiles, that and the space it takes up are really the only drawbacks....or if you have a virus on your old windows, it can carry over dat BS.
edit
i saw a screenshot of the install screen and it is identical to the Windows 7 reinstall screen, you have all the same options for deleting partitions, formatting the HD, or not.