DHCP, company runs out of IP addresses, smoothest way to define a wider range

Hi,

My issue is pretty easy to explain:

The company I joined has an established network for like 30 years in 192.168.0.0/24 (I know, that sucks). This is the site #1.

The DHCP server is Windows Server 2012 R2 and is setup to deliver IP adresses ONLY on this range.

With this kind of bad design, we will soon be out of available IP adresses to deliver.

Question is, what would be the smoothest way to make the range wider, say 192.168.0.0 /16 without having to reconfigure many things...

My first idea was to simply assign that range 192.168.0.0 /16 or 255.255.0.0 instead of that one 192.168.0.0 /24 or 255.255.255.0 in Windows DHCP server but some machines like printers and stuff have 255.255.255.0 manually set to it, and all of the VPN (FORTINET) is setup like that on all sites :

Note that the company has other buildings, with their own DHCP servers but reachable from site #1 via VPN.

Buildings #2 to building #8 are 192.168.2.0/24 to 192.168.8.0/24

So knowing all of that, now I think that would be way better to create a second 255.255.255.0 range assigning from say 192.168.10.0/24 as it would be way enough for the "small" company devices, and will not break everything else, I guess, but I have NO IDEA how to do that...

Thanks for your help!