You need the cost of debt and equity to arrive at your WACC. Does the cost of debt and equity change each year? What could be the reason? We address this question on this page.
The cost of equity and cost of debt reflects the risk profile of the firm and capital (debt or equity). As firms move from one life cycle stage to another, the firm’s risk profile changes. Therefore, the firm equity risk and debt risk will change, causing the cost of equity and cost of debt to change. Therefore, you must reflect the change in debt and equity costs in your DCF.
For examples of different discount rates being applied each year, see Prof. Damodaran’s valuation models on Apple or Amazon. Look in row 12 of the valuation output tab in each of these models.