Technically, yes, you can. But, as I say in my books, I highly recommend you do not. It almost alway leads to buggy behavior.
I recommend you finish your version 7 project in version 7 and start a new project in version 9.
I think you may have read that the wrong way round. The question is whether or not a PRE9 file can be opened in PRE7, not can a PRE7 file open in PRE9.
So RacheySC92 the answer is No - you cannot open a '9' project in '7'. You may open a '7' project in '9' but it is not at all recommended and could go bad on you at any time.
Insanity is hereditary, you get it from your children
Thanks, Neale! You're exactly right of course.