Turns out it's worth being concerned about, or at least prepared.
Luckily there's plenty to get you started (from MS):
Unpredictable behavior if you migrate a roaming user profile from Windows 8 to Windows 7
Couple blurbs from that KB article:
Windows 7 and Windows 8 use similar user profile formats, which do not
support interoperability when they roam between computers that are
running different versions of Windows. When a user who has a Windows 7
profile signs in to a Windows 8-based computer for the first time, the
user profile is updated to the new Windows 8 format. After this
occurs, the user profile is no longer compatible with Windows 7-based
When a user who has a Windows 7 roaming profile signs in to a Windows
8-based computer for the first time, the user's roaming profile is
updated to the new Windows 8 format. The Windows 8 user profile format
is incompatible with Windows 7-based computers.
If a user must switch between Windows 7-based and Windows 8-based
computers, then you must configure the Set roaming profile path for
all users logging onto this computer Group Policy setting for the
The full KB (obviously) contains more information and how to deal with it.