In a later cake recipe, can I substitute a 13x9 pan? How does cooking time change%?

I don't know your original recipe but there are conversion charts for increasing and decreasing pan sizes. It usually involves some basic multiplication. Scroll down to "Adapting Pan Recipes to Fit a Certain Cake Pan" or google a chart. You'll also have to adjust the baking time, usually longer if you increase the batter and use a bigger pan.

https://sallysbakingaddiction.com/cake-pan-sizes/