Device and Users question

is it too late to switch freepbx to device and users mode AFTER rolling out about 10 phones or is that ok to switch the modes at anytime?

lets say i switch freepbx to Device and Users mode, what will happen to the existing devices and users, will they get erased or will they continue to exist?

also, will i need to create seperate device accounts on the system?

any documentation on this would be great.