It is possible to change the PHP version for a website through admin.savvii.nl.
Log in and click on the 'manage' button for the website you want to change the PHP version for.
On the tab 'Overview' you can view and change your PHP version.
After changing the version in the dropdown menu, you will see a screen with some important considerations for chaning a PHP version. Read them carefully and click on 'Confirm' to start the version change process. This process takes about 5 minutes. Test your website when the new PHP version is active. If your website does not perform correctly, you can revert back to your former PHP version using the same procedure.