This is not an official answer, but I can tell you of something that worked for me.
You can start geth separately and use command line options geth cli options to make it start on your preferred network.
You can change the default network in your Mist/Ethereum settings. I don't know if it is recommended, but I did and it worked.
For me the network file was showing "rinkeby" I had to restart my mac to open mist on rinkeby at start again :/
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This is not an official answer, but I can tell you of something that worked for me.
You can start geth separately and use command line options geth cli options to make it start on your preferred network.
You can change the default network in your Mist/Ethereum settings. I don't know if it is recommended, but I did and it worked.