You might have had a company setup with the classic 192.168.X.X and need to move them to something less generic mainly so people can VPN from their home connections on routers that don’t support an in between subnet.

When you can’t create another Vlan : 

1. Change DHCP Server Leave to Minimum like 1 Minute
2. New IP changes to the primary adapter ( if set to DHCP  ) 
3. Add IP from the old subnet to the Secondary IP for the adapter ( So it can still access devices on static address’ we will find out what these are in step 5) 
4. Wait 
5. Check ARP Cache for the old subnet ( arp -a )  and find old devices statically assigned like printers , change these devices over to new subnet
6. Continue until nothing on the old subnet
7. Remove the secondary IP from the secondary IP of interface
8. Restore DHCP Server Lease