The question is based on a misconception that "Curry Powder" is the dried crushed form of the curry leaf. It most definitely isn't. The question comes up onto the site frequently through searches

The curry leaf comes from a specific tree and the leaves are generally used fresh. This has the vaguest similarity to various curry blends but it can't be substituted either way. Curry powder is a blend of various spices. There are many dozens of recipes for different types of curry powder and they all tend to have a short shelf life after being roasted, ground and mixed.


These powdered spices are typically used for blending curries or masalas

The "western curry powder" is a British inspired creation during that period of the British Empire controlling the Indian sub-continent. and I think was created to allow residents to take home the flavour of India. There are a few long standing brands of this and they have been highly successful for adding to various recipes. These are also used in other places like Malaysia and other areas of South East Asia. There is little variation between them but they are certainly aromatic and spicy but would not suit most recipes from India, Bangladesh Sri Lanka or Pakistan. It seems that all curry blends or masala or at least the majority have as their primary ingredient, ground cumin seed, turmeric and coriander powder. After these three the balance of spices varies greatly in number and kinds.

There are masala with curry leaves as a component of the blend. These tend to be called Curry Leaf Podi, I have two recipes for such listed. A point about curry leaves is that dried they lose their flavour very quickly. Curry leaves can be stored frozen with a usable period of approximately 6 months. You can sun-dry curry leaves for immediate use in the curry powders i mentioned . This seems like a contradiction. The issue here is that fresh leaves are quite "sticky" when it comes to force drying, grinding and blending them with other spices and the roasting of the leaves is difficult for that reason. The compromise is to dry the leaves in full sun for a day just prior to using them in a powder blend. In the sealed blend the flavour is minimally stabilised but I have no information on the stability period of the curry leaf containing masalas.

Curry leaves are generally used fresh and most often not part of a masala but rather an enhancement or a separate ingredient in a dish. Sometimes as basic as laying a sprig or a few leaves on the cooking food. The flavourant in curry leaves is an "essential oil" and it seems that it is very volatile meaning that it evaporates very quickly and the flavour is in that oil. There may be other aspects affecting the loss of flavour also, things like unstable flavour when exposed to air and oxidation of the flavour product.