There is an option to delete previously scanned file IDs (this may not be the correct term, I don't have access to NIS 2014 right now). Would this allow a full scan of all files?
Yes there is an option in Settings > Computer > Antivirus and Sonar Exclusion > Clear file IDs excluded during scans < Clear All and the Help info on that setting says the following:
Clear file IDs excluded during scans
Lets you clear the hash file IDs of the files from the exclusion list.
You can use the Clear All option to clear the hash file IDs of the files that are excluded from scanning.
Exclusion based on hash file ID is the default method of excluding a file from scanning.
When you clear the hash file IDs, Norton Internet Security includes all the files in your computer for the successive scan.
I would think if this meets you desires - it would have to be done everytime prior to running a manual scan?