Question : DHCP - Addresses running out

We have too few addresses in our current scope, Class "C".  

We use:  192.168.3.x      192.168.5.x    192.168.19.x  

But with a 255.255.255.0 subnet.   I know that I can change the subnet and allow more addresses.   My question is what should I change the subnet to to make the most possible addresses for each subnet?  And what will I need to consider when I change this beginning with the DHCP server?

Besides the obvious; changing subnet addresses on all statically assigned devices, changing firewall and cisco router entries.   I'm trying to define what "could go wrong" or what I might be overlooking doing this over the coming weekend?

Can anyone who has done this please give me some hard won advice?  The network is only 400 users but I don't want any surprises.

Answer : DHCP - Addresses running out

yes. you can keep using 192.168.19.x addresses, but you will need to change the subnet mask on those machines (like a printer at 192.168.19.25) before they will be accessible from a new machine at 192.168.5.12 for example.

I would avoid mapping a 10.x.x.x class "A" subnet. Before you get very many more machines on a single subnet, you will want to split to multiple subnets anyway, and route the data. The broadcast traffic will be a performance problem soon if it isn't already.