Here is a reply from the start 9 team;

Switching between the two is a little complicated because of the way this is temporarily set up to allow both to exist and share the same blockchain. Because they do share it, definitely don't uninstall anything. The trick is to get access to downgrade and reinstall options, each of which might be relevant depending on what versions and upgrades are available.

Basically...

Reinstall method In the StartOS System tab, click the “Manage” heading 5+ times to enable developer mode.

Visit the Marketplace, and the version you want to install. Click “Reinstall” on the service, or

If you get a notification claiming the install failed. Don’t worry, it succeeded. You just need to open the config and click “Save”.

Downgrade method (should not be needed unless the versions are out of sync) In the StartOS System tab, click the “Manage” heading 5+ times to enable developer mode. (Do this anyway if you'd a switcher, might be useful later) Visit the Marketplace, and the version you want to install. Scroll down to Versions and select at least one version down.

Install that.

There'd be no need to resync, and all dependent services should be happy.

In StartOS 0.3.6 this will all be unnecessary, and the process of switching between Bitcoin implementations will be streamlined.