How to Prevent Permalink Change when add a 2nd Category to a Post ?

Hi! I wonder if somebody has a solution for it. I use category based permalinks and I like it that way. Sometimes though a post has to be included in 2 or more category for display purpose (as an exemple, my theme does not support tags for some features). So I edit the post but OOPS! If the 2nd category slug is alphabetically ordered before the original category, the permalink change to the base of the 2nd category! Horrible! Is there a way to prevent the category-based permalink to switch over to another one? thank you!