If in doubt, switch back to the core Boost theme before you upgrade.
You'd be unlucky if the upgrade broke so badly that you couldn't use the site. But even then overriding the theme can be done by adding
$CFG->theme = 'boost';
...in your config.php file
You'd be unlucky if the upgrade broke so badly that you couldn't use the site. But even then overriding the theme can be done by adding
$CFG->theme = 'boost';
...in your config.php file