As a workaround that’s easy to reverse you could use a symlink:
ln -s /opt/rh/rh-php73/root/usr/bin/php /usr/bin/php
This would give you 7.3 output from the command
php -v. But it doesn’t change how global/default PHP is set. Also consider how it affects 5.4.
Normally in a multi PHP setup, global PHP is the installation configured by /etc/php.ini no matter the version. To make 7.2 or 7.3 global, 5.4 would need to be replaced. You won’t be shifting things around in Virtualmin, you’ll need to upgrade or replace 5.4 with the newer version in a terminal using packages from SCL repository. I’m not much help with installing SCL PHP packages because I’m accustomed to Remi’s repo.
Let’s say you decide to take the long road and replace 5.4 with 7.3. First thing is to remove existing 7.3 because you don’t want it installed twice. Then, if you still need 5.4 you’ll have to reinstall it as an extra since it’s been replaced by global 7.3; 5.4’s install locations will be different than before and its .ini file would be /etc/opt/rh/rh-php54.ini instead of /etc/php.ini. The net benefits from juggling PHP installations this way won’t amount to much.
I bet the symlink solution is sounding pretty good at this point.