You need to use the Terminal to start the upgrade process. The App Store now tends to download a stub installer which when run will download the full High Sierra installer. Once this is done, just quit out of it and run the following command which assumes the installer is located at the default location in your Applications folder:
/Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/startosinstall --converttoapfs NO
I'm not sure if the command I'm providing will work from the stub installer or requires the full installer. Most likely only the full installer. You may want to Google it a bit so you feel more comfortable about it.