Old and new NAS issue

[HolmesLane]

Previous set up: the organisation has a NAS with 2 shares under an IP address ending 50.3

New set up: a new Buffalo NAS using a different name but connected to the same network. The Linkstation allocated a new IP address ending 50.25. The old NAS was unplugged and has no electrical or network connection to the system. For continuity purposes I want to change the IP address of the new NAS to that of the old one, but Linkstation software says that it is already assigned.

Issue: it is still possible to open and save documents using the existing mapped drives to the old NAS, even though the drive isn't connected. How can I remove these mysterious connections to the old NAS, and change the IP address of the new one to 50.3?

[Texturtle]

If you ping the 50.3 address do you get a response?

What is the network topology? What OS is running on the clients? Local DNS server?