sure, I think so, as long as your willing to sign your name to it. It is a great thing to update the schools on when you're writing secondaries or waiting to hear from them. If you end up not going, you should probably let the schools know so they don't think you were just trying to pad your application.
How about significant experiences in high school? I assume that is probably not a good idea... but I volunteered at a nursing home for 2 years in high school.
The AMCAS application has section that is for Post-Secondary Experiences, meaning experiences after high school. However, it seems reasonable to include experiences from high school in your personal statement if appropriate.