It might be something we add a UI for eventually, but the current process (and therefore the process people will use during the dev preview) is to use the CLI (which you will need for the install anyway).
Just to be clear; the installer will automatically apply utf8mb4 if it is supported for clean installs, but, again, the CLI will be needed during the dev preview if it is installed before your environment supports utf8mb4.
It's also worth noting handling this in the upgrade is problematic as we can't necessarily distinguish between core XF tables and those added by add-ons, and we can't always guarantee a table added by an add-on will be convertible without error.
Just to be clear; the installer will automatically apply utf8mb4 if it is supported for clean installs, but, again, the CLI will be needed during the dev preview if it is installed before your environment supports utf8mb4.
It's also worth noting handling this in the upgrade is problematic as we can't necessarily distinguish between core XF tables and those added by add-ons, and we can't always guarantee a table added by an add-on will be convertible without error.