To a point if you are very careful. The machines using the old subnet mask will not be able to talk to the machines using the new subnet mask. You would have to be very careful not to allow your dhcp server to give out any ip in the old block or you would get duplicates. You likely would have to assign the new router a IP that not the fairly standard lowest address since it would overlap your current one....it depends where you pick your /22 network to start.
There are lots of nasty tricks you can play with static ARP and such also but I would change the mask as quickly as possible. You can leave the IP the same as long as you ensure it won't duplicate.