On the properties page for the least preferred network: Tick "look for other wireless networks while connected to this network" on the network you don't want to be connected to, when there's a more preferred network you'd rather connect to instead (and that you're computer has connected to previously).
It's important to also make sure that the networks are sorted into your preferred order as Windows will connect you to your more preferred network only if you have set the order in Network and Sharing Centre "Manage Wireless networks" list. The list had drag-and-drop so you can arrange the networks easily.
In layman's terms: "I'm connected to this network but I'll keep looking for other networks I'm aware of. When a network on my list is higher than this one connect to it instead".