A recipe calls for 5 Tbsp chili powder. I only have Hot. No more medium. Would you just put in less or make the trip to the store?
Start your substitution on the low side - use 1 Tbsp hot chili powder in the recipe, cook, then taste and add more if or as needed.
You can always add more, but removing too much heat is near impossible, or requires bulking up the whole dish.
No need to go to the store - you can always adjust amounts of things like chili powder for your heat levels preference. But as mentioned above, definitely err on the side of caution - start with a little and build up from there, tasting as you go.