Coupon rate on a bond doesn’t change. What changes is the market. The market prices different yields over time but when you purchase a bond the coupon rate on that bond is locked in.

What makes the value of the bond go up or down is the discrepancy between the coupon rate of that bond versus what rate you can get on the open market.

This bond you got at 10% yield moons in value as the market rate goes much lower. You would pay a lot more for a contract yielding 10% coupon when the market rate is like 3%, right?

Conversely if you bought bonds with a near 0% coupon then the market goes to 5% yields, your bonds are cratered in value cuz the buyer can get such a better return buying new bonds on the open market.

This is how some of these banks are getting rekt as they loaded up on “risk-free” bonds when the yields were close to zero, then the Fed jacked rates up. The bond market prices the yields higher and low yield bonds get rekt.

That’s my understanding anyway.