The new PHP version will not be enabled until you purposely do so. Upgrading to PHP 7.2 using the service panel will be made available some time next week, but until then you can get in contact with support and ask them nicely to change it for you.
Magento & PHP 7.2
Keep in mind that there currently is no stable Magento version yet that officially supports PHP 7.2. In the upcoming Magento version 2.3, PHP 7.2 will be supported for the first time.
Magento, with assistance from our community, is implementing PHP 7.2 compatibility for the 2.3.0 release. Any backward-incompatibility issues will be resolved in this release, and all 3rd party libraries now support PHP 7.2. Fully tested 7.2 support will be delivered in following patch releases.
For more information about Magento 2.3 see the Alpha release Magento documentation. This new Magento version is expected to be released somewhere later this year, but the exact release date is still unknown at this moment. If you want to test out the Magento 2.3.0 alpha already, check out this guide by Fooman or take a look at the 2.3-develop branch on github.
New PHP features
PHP 7.2 will offer some performance improvements over previous versions, but it also has some other nice new features like extension loading by name, abstract method overriding and a new ‘object’ type for type hinting. Notable as well (especially because of Magento’s reliance on mcrypt) is that the Sodium cryptography library has now become a core extension. For the complete list of new features see the migrating from PHP 7.1.X to PHP 7.2.X guide.
Benchmarks by phoronix have estimated the performance increase to be around 8 to 12% (depending on the workload obviously). But because Magento does a lot of XML processing it is not unlikely that those types of numbers will also become a reality in production environments for CPU-bound pages.