Is Miley replacing Selena as Demi's BFF?

Miley and Demi have been tweeting BFF love notes to each other like crazy lately! And during a trip to London, they were inseperable on the red carpet. So, is Miley Demi's new BFF? The girls are tight- but she's not replacing Selena, 'cause no one can! Demi dishes about Sel, "No matter what we go through, nothing and no one can change our friendship!"