I'm changing my network from having every device on flat network to using VLans. My problem is that we already have a lot of devices on this network(192.168.20.0/24). From theory, I read that each Vlan has to be a different subnet and then I need to configure virtual interfaces on my Cisco router to cater for inter vlan routing.

1) How can I segment this network with minimum down time on the devices already on the network?

2) Can I just create Vlans and leave all these Vlans in the same layer 3 network so that they can go out of the network (I am not too concerned about inter-Vlan routing) or I have to create subnets which means reconfiguring the existing devices (something I do not want).