That is why they are called pre-defined scans, that way you have a known condition for comparison or debugging results, etc. and why you can create a custom scan, which you can change to your hearts content. As Igor mentions if the scan settings aren’t greyed out then it is unlikely to be a pre-defined scan that the same settings can be changed.

If you happen to get unexpected results in your custom scan, then you can simply delete that scan and create a new custom one, this is somewhat different if you could change the pre-defined scans as you can’t easily change it back to the start status.

So just create a custom scan if there are areas that you wish to change, that custom scan can be given a more meaningful name and is saved, available for use in the future.

Personally I don’t feel there is a need to use anything other than the Quick or Full System pre-defined scans, as for the most part on-demand scans are going to be scanning files that are otherwise dormant or inert. If they are active, not dormant or inert, then the resident on-access scanner will scan them before they are, created, modified, opened, run or executed.

So the resident on-access antivirus, depreciates the need for on-demand scans, I just do a Quick scan, scheduled to run once a week.