Hi Bonnie, have you tried searching the forum for posts about altitude baking? If you put the word “altitude” in the search box and click search, you’ll find a number of posts listed incuding yours now! They include general advice, not specific to Hamentashen. But you might find something useful.
Checked my baker’s manual. Basically there are two major types of ingredient change needed to successfully convert recipes for baking at higher altitudes: 1) decrease leavening factors (baking pdr, baking soda, yeast); and 2) increase structural factors (eggs and flour). Creaming time can be reduced and oven temp increased. “How much?” is the question, and the answer is, “It depends.” Mainly on how high above sea level you’re talking. Complicated, huh?
My hunch is that you should bake a small batch as you normally do, and see what happens. But is there anybody you can consult who lives in the area you’re going and bakes excellent Hamentashen? That would be the quick way to a reliable answer. Good luck. Let us know how you make out.