I have a static lease defined for my OBi110 - that is, the DHCP server always returns the same IP address for the OBi110. At one point, I changed the static lease on the server to use a different IP address, but did not reboot the OBi110. At the halfway point of lease expiration, the OBi110 attempted to renew its lease on the old IP address, and the DHCP server responded with a NAK, since the IP address had changed. That is expected behavior. However, the OBi110 did NOT request the new address after receiving the NAK. It repeated the DHCPREQUEST for the old address 5 times, to which the server NAKed each time, and then gave up and continued using the old IP address.
There are, of course, ramifications of changing the IP address - if calls are active, they will most likely be dropped, and so on. However, the OBi110 should be requesting a new address after receiving a NAK from the DHCP server.
The lease (from the OBi110's perspective) hasn't expired yet, since the lease expiration is 1 day, and it's only been 13 hours since the lease was originally obtained. So, I'm waiting to see what happens at that point... if the OBi110 continues using the address even beyond expiration, or if it finally requests a new address at that point.