Just because you are safe, she isn't. Moving from the vagina to the anus would not require a change however don't forget to lubricate before entry. The moisture from the vagina would not be sufficient for penetration. It is probably wise to acquaint yourself with the art of enema. The use of an enema prior to anal sex makes life a lot easier especially for the partners who are embarrassed by both the smell or the sight of their bowel contents on the end of someone's cock.
Enemas can be fun and can be made part of the anal play.
Just be considerate and aware. Whilst on this subject of embarrassment quite often, especially if the thrusting gets quite fast and deep your partner may find a build up of wind within the bowel. Lovely lying there wrapped within one another's arms enjoying the quite moment of orgasm to have it interrupted by a fart. But hey it is part of it, learn to laugh, a lot, on average women fart 7 times a day men 12. To those who are used to vaginal farts this will not be a problem.