Acontece que vale a pena se preocupar, ou pelo menos preparado.
Felizmente, há muito para você começar (do MS):
Comportamento imprevisível se você migrar um perfil de usuário móvel do Windows 8 para o Windows 7
Algumas sugestões do artigo da base de conhecimento:
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 computers.
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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 computers.
O KB completo (obviamente) contém mais informações e como lidar com isso.