Stephen
I have been thinking about this problem too, as it not only applies to OpenSprinkler pi but also all the other raspberry pi systems I have now built using fixed IP addresses.
The main problem is the fixed IP address that we used for the user-interface. If you only had the one OpenSprinkler then I suggest the simple answer would be to leave that device in DHCP mode and tell your new home owner to use the OpenSprinkler App. As when he first uses the App it will automatically finds your Controller.
The second issue is your WiFi connection and its Network SSID and password.
The suggestion offered by Samer and Ray would be to set up a WiFi Client and connect your OpenSprinkler to its’ Ethernet port… which still needs some intelligence from your new home owner to understand how to reprogram it to his wifi network and also I don’t think this would solve the problem.
I have now found a solution that works by using a similar type of device (but not the same) as Samer and Ray suggested but in AP mode… here’s the details of how I setup my Wifi AP adapter which has a Ethernet WAN and as a bonus a LAN port.
Power up the adaptor, plug an Ethernet cable into it and connect that cable to your home router and then follow the manufacturers instructions on how to set it to AP mode. (My one instantly came up in AP mode when I pressed its rest button). The adapter then sets up a “open” wifi network that you will see on your computer or mobile device (you will see it as a unsecured wifi signal as stated by the manufacturer instructions).
My new wifi network was called Wireless-N and on the back of the adapter was its IP address as 192.168.10.253 and its username and password. Using those setting I logged into its user interface. And then changed its SSID to “Home Controller” and gave it a new password.
I then changed back to my home WiFi network and SSH into each raspberrypi and changed their static IP address for wlan0 (the wifi connection) only. I left the eth0 (Ethernet connection) with it current setting just in case I screws up with its wifi settings.
Wlan0
Address=192.168.10.122 (the new static address of my pi)
Router=192.168.10.253 (the AP router address also known as the Gateway address)
Domain name servers=192.168.10.253 (you might need to set this)
Then changed back to the new wifi network.
Next open up OpenSprinkler App and manually connect to your controller and change the IP address to its new address.
Problem solved.. you now have a OpenSprinkler and in my case a home automation system on its own network.
So when you sell your home, leave this adapter with its Ethernet cable for the new owner to connect to his routers LAN port. And give him the SSID and password and tell him to use OpenSprinkler APP as you do.
Tip… be aware not to install this adaptor to close to your home router as it will weeken the strength of both WiFi signals.
Now back to the Ethernet connected OpenSprinker
As above for the wifi connected devices and now change the static address of your OpenSprinkler and connect that to the LAN port of the WiFi AP adaptor. Or use another adapter in Client Mode.
Hope that helps.