lan expansion

Hi,

Now my lan is too big to use the 192.168.0.0/24, so I plan to upgrade to the
192.168.0.0/16, for the Accounting department 192.168.1.0/16, sales
departmnet 192.168.2.0/16, and so on.
It seems a lot of work to change so many client IP address.
I am wondering if I change the mask from 255.255.255.0 to 255.255.0.0, will
the lan communication be slower? And is it possible for the client
192.168.0.1/24 to talk to client 192.168.1.1/16 without using routers?
Any help will be appreciated.

Jason