Just properly 301 redirect all old URLs to the new URLs and you'll be fine; remember you'll see a slight change in SERPs for a temporary amount of time... you will retain your original Pagerank during the next update or the one following that.
That depends. Most of the time it's temporary, as ssandecki said. However, there is no time period that's fixed in stone, such as "X amount of hours" or "Y amount of days" or "Z amount of weeks".