If you were playing golf, I doubt if an “exact patch of grass” question would come up. If you were planning to make Leek Potato Soup, you do not need permission to make Scallion Potato Soup. In practice, it would probably sell as well at your restaurant. I would not advocate calling your soup Lobster Bisque if it was devoid of lobster. You could get away with Mock Lobster Bisque if it reasonably tastes like lobster.
Speaking of lobsters: lobsters are trapped throughout the year, so there is no “best time of the year” for Maine lobsters. On the other hand, seasonally sustainable cooking means offering patrons what is best THIS time of year without incurring environmental damaged caused by shipping food great distances. No excuses are necessary as you are doing them a favor. They get the best at an affordable price. This works equally well in you home. Who has a greater need to be frugal the restaurateur or the home chef on a fixed budget? Local, organic is better for us in either case and helps sustain local healthy food vendors.
Blogging and feedback on recipes-reviews show some people are still shy about altering many recipes. I would like to say that except for baking, substitutions are almost 100 % successful. Often they result in an improvement. I was noticing the Libby Pumpkin Pie comments where one home chef used Cream instead of evaporated milk. Sounds delicious to me.