Unless you have routing setup for that old IP address range, you're not going to be able to get to it over the network. Do you have a DHCP sever on your network? If so, and you've got the internal jetdirect card, then go through the front panel setup and set the printer to DHCP. Shut off the restart the printer and it will be accessible from JetAdmin again and you can then set the static address.
If not, then you're going to have to take one workstation, move it back to the old IP address range, install the JetAdmin software on that machine, set the ip on the printer, then move the workstation back to the new addres range.