Discounting using a marginal cost of capital

I am working on a DCF model for my company and was struggling finding any information on debt amortization and the changing in the marginal cost of capital.

For the specific project I am looking at, we would be paying down principal on the debt for the project, thus changing our marginal cost of capital for the project (the weights). My belief is that I should be changing the discount rate (marginal cost of capital) each year as the weights of debt to equity change. This would should an increasing marginal cost of capital until the debt is paid off and then growth would be constant.

What are your thoughts on this and does anyone have any literature or resources I could use to research this more in regards to marginal cost of capital and debt principal paydowns?

If the capital structure changes throughout the life of the project, use the Adjusted Present Value (APV) model instead.