Este é um problema conhecido com arquivos off-line no Windows 7 e no Windows Vista.
A solução oficial é renomear seu arquivo para um nome diferente, depois renomeá-lo novamente para o nome original com a devida capitalização.
You cannot rename files by using different case when the files are made available offline in Windows Vista or in Windows 7
Action
When files are made available offline in Windows Vista or in Windows 7, you cannot rename files by using different case.
For example, you have a file that is named TEXT.txt, and you want to rename it to text.txt. However, you will be denied if the file is made available offline.
Cause
The Offline Files feature uses the client-side cache (CSC) database. When the files are made available offline, the CSC database prevents users from changing the case of file names.
Resolution
To work around this issue, rename the file to another name, and then rename back with the changed letter case.
For example, rename TEXT.txt to word.txt, and then rename word.txt to text.txt.