During the development process, code may be tested on an iterative basis—typically at specific milestones—by quality assurance engineers.
The test environment should emulate the production environment, to ensure quality throughout the product life cycle. Any defects are reported back to developers, to be fixed. This iterative development and testing cycle continues as long as necessary to produce a high-quality product.
As part of the testing phase, you may need to upgrade the test environment itself as the project develops. During the transition, you may need to consider upgrade scenarios as part of the testing process.