Categorizing posts changes URL – management ideas?

I looked for answers on the forums, so plz direct me if this was already answered? Thanks.

Is there a way to manage which parent category shows up in the URL if there are a # of categories for a specific post?

For instance, I add categorize a post as “family” and “holidays” (both parent categories) and when I save it, the “family” shows up as the parent category in the URL like so: mysite.com/family/postname — when I’d like it to be mysite.com/holidays/postname

Also, if I categorize a post and later change it, I see the URL changes based on the new category. Does this mean the “old” link is broken? If this is the case, the above question is so important, as I’d like to add more categories to the same post at a later time, but if this breaks the URL, this would prevent me from doing so.

Any help is appreciated.