Some of these factors alone may dictate one strategy over another. Regardless of which method is employed, a successful upgrade to SQL Server 2008 should be smooth and trouble free. To achieve that smooth transition, you must devote sufficient planning for the upgrade and match the complexity of your database application. Otherwise, you risk costly and stressful errors and upgrade problems.
Like all IT projects, planning for every contingency and then testing your plan gives you confidence that you will succeed. However, if you ignore the planning process, you increase the chances of running into difficulties that can derail and delay your upgrade.