I don't have one to test, so have you tried to change it and upload to the system
in the picture attached it shows the steps to converting the file. if you do that, when it opens check it has changed.
Up load and see if it accepted the change to IP mode
After saying all that, the original question is how do I change the system back to ip-mode.
When I looked at the bigger picture, the fact is you can't have more than 256 IP - PT phones on one NS1000.
To have the 640 IP-PT phones, it has to be across more than one NS1000 in one look network.
So the short of it is, even if it shows 20 slots, you still can only use 8 IP extension cards on one system.
640 is the max of extensions you can have on the 1000, but that depends on the type of phones and gateways attached.