Well, this is one of those rare lazy weekends where we have no plans so we watched the disc we received this afternoon, took it up to the local store about 5 minutes away, exchanged it for another new release not yet available on NetFlix and by the time I got to my car I had an email that showed the confirmation of the exchange on my account. According to the store employees, when I turn this exchanged disc back in, BlockBuster will ship the next movie in my queue without waiting for the other one to make it back to them.
Just thought I'd update my experience for anyone who is considering the change.
As long as you have an open slot, if you use Total Access renting from the store is the same as renting on-line you can keep the movie as long as you like however the movie will take up on of your slots.
The only time the return date applies is if you have used your limit of in-store exchanges. I also don't believe you can return more than 2 at a time for your free exchanges. They will give you 2 and make the other a normal rental.
Play close attention to your queue, if the disc does not appear in your queue when you rent you are on rental terms and pay attention to the due date as to not incur late charges.