What happens to changes while backup is in progress?

Changes will take affect in next backup.

Once a backup has been started the policy configuration has been read, and any change will first take effect at the next backup.

Netbackup scheduler wakes up with 5 minutes intervals - any policy change with 5 minutes from backup windows opens is a timing race.

Case 1:

This means that the files are open during the backup. Here you should differentiate from Unix & Windows clients.

For Unix clients it works by default, you don´t need to do anything and the file will be backed up with the changes, if the changes end before the backup ends. Only if you see a SC 1, you can manage or configure the attribute 'busy file action', or trough the Host Propierties. 

For Windows, you should configure the 'Open File Backup' under Host propierties, where you have different options to configure, but the behaviour is that a snapshot will make for the open files during the Backup, this is the different options to configure:

You can find the detailed information here (page 98):

http://www.symantec.com/docs/DOC6452

Case 2:

It was answered in this thread. To resume, if you update a policy during the backup is in progress it will not affect the backup in progress. If you edit the policy before the backup execution the changes will take efect if the 'Policy Update Interval' (Host Propierties) has been done before the execution, or if you execute the command for update the policies manually.