1. Start adding a release by selecting "Add Release" under "Editing" on the MB top menu bar.
2. Select any existing release group to add the release into.

Now you can still select new release type(s), though you should not be able to (weren't before extra types were added). Luckily at least changing the main type doesn't actually stick and go through editing process, haven't tried (de)selecting any extra types. Still, selecting the type should not be allowed through the release editor for an preexisting release group.

Note: you can't select the types, if you start adding the release from release group page using the "Add release" button on the sidebar.