Well, first of all you have to know what's the difference between these security methods:
- NTFS means that QlikView will lookup for the Windows Users and Windows Security file permissions in order to control user rights to get a CAL (Client Access License) and to be able to access applications. So for an user to be able to login to the QlikView AccessPoint it will need to have a valid user account (local or domain), and in order to be able to interact with apps this account must be granted the permission to, at least, read a qlikview app (.qvw file) in the Windows Security properties of the file.
- DMS means that QlikView will control the right to get a CAL and the access to apps disregarding Windows Security users and file properties. In this scenario you'll have to manage your own users and provide user's access to apps from the QMC (QlikView Management Console). This information will be stored in the ".meta" or ".shared" files (I don't remember which one) that appear right next to the .qvw files.
So, if your server is running under NTFS you're using Windows file system and security to control CAL assignments and user's access to applications, which is the most common environment. I don't recommend to you to change to DMS unless you want to separate QlikView management from Windows security users and permissions.
Obviously if you want to change to DMS you'll have to setup a maintenance window in order to create all users, cal assignments and user access to apps before going to live again.
If you change it to DMS mode then you have to re assign the Cals to particular users to access.
difference is if NTFS security is checked Users who have Doc cal still can see all the application but can't access and if DMS security is checked then users having Doc Cals can only see their applications whom they have access but users who have Named cals you have to re assign the casls similar you do with Document cal users for this you have to go in QMC -->User Documents Authorization Tab which get highlighted when you select DMS mode.
may b helpful