Problem you run into is how will site B box Y know you moved 192.168.18.X to the other site, if Y wants to talk to X.. He will think its local, and never send to its gateway.
And as you move X over to A, how will X know that box Y is still over at site B..
Easier solution would be to use a different network say 192.168.19 or 172.16.18 as you migration network.. So you move 192.168.18.X to site A and change its IP to 172.16.18.X.. Changing any dns you need to at each site to reflect the fqdn new IP on the new site.
Once you have everything moved over - you can change the 172.16.18 back to your 192.168.18 network.. And change all your dns to reflecting the correct address.