Louise,
Yes, the IP address can be changed without manually resetting the FP-1600.
A typical cause of being unable to find devices is having a duplicated IP address. Considering that you do not have problems locating devices with the first IP address and that you do with the second, are you sure that the second IP address is not in use? Prior to assigning the FP-1600 a new address, try pinging the new address first. If you receive a response to any of the packets, you do not want to use that IP address.
Regards,
Aaron
LabVIEW Champion, CLA, CPI