What happens if budgets change for ongoing projects?

If budgets change anytime for ongoing projects, new budgets can be created based on the old or existing budgets. This retains all the budget changes. If you need to see any data concerning the changes in budgets, you can run reports using all the budgets instead of the actual spent against the projects. Budgets don't affect the overall metrics of projects; they are used for comparison, to gauge whether you are overspending or underspending.