You can't alter a signed file without invalidating the digital signature. Otherwise, they'd do a pretty poor job of confirming where the file...
One of the functions of a digital signature is to confirm that the signed file hasn't been altered in any way, so I'd consider a hash value...
Try raising the Alert Frequency Level to Very High. http://help.comodo.com/topic-72-1-170-1731-Alert-Settings.html
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