We are currently running PHP 5.5 and have designed a test area using Magento 2. If we upgrade the server to PHP 7 and make sure all the modules are installed, would we need to reinstall Magento 2? Or will Magento immediately start using PHP 7.
If you upgrade php to php 7 then there should not be any issues with Magento 2. But if you are running the Magento 1 also on the same server it may create issue as Magento1x officially supports up to php 5.5.
It is better to implement the changes on the development environment first.(Do not perform directly on the Live site).