Expand Private Network Address Space?

Hey everybody, I've got a question about extending our private IP space...

So we are currently using 192.168.10.XXX with 255.255.255.0 as our subnet.  We've grown enough that we need more than 256 IP addresses (or at least we are very close) and we would like to extend our address space with the least amount of effort.  I would like to not have to re-IP every device that has a fixed address right now.  What options do I have?  Different subnet mask?  Some kind of routing thing? 
Near as I can tell, one option would be to simply change our subnet mask on all devices to 255.255.248.0 and that would leave the existing IP addresses as they are, and it would give us 192.168.8.1-192.168.15.254, which would be great as that's centered around the 192.168.10.XXX address range.

But I'm sure it's not that easy.  Thoughts, ideas?