Upgrades have been known to happen, but are extremely rare.
My fellow DVCer Aunt reserved 2 studios for 2 nights on points for her sister (also my Aunt

) for her family of 6 last August at the VWL. When they got there, they were upgraded to 2 one bedrooms!!!

The resort and rooms were so nice that her kids never even wanted to go to the parks. They just wanted to stay in their rooms and swim in the pool.
The upgrades were not requested. She was told that they were given the upgrades due to the studios being oversold.
I have a feeling that the only time that they give upgrades is due to inventory control. Due to the points system, this would be rare. Maybe an existing cash guest wanted to stay an extra night or so in a studio. There was inventory to upgrade the DVCer staying on points, so the resort let the cash paying guest extend their stay with cash in the already reserved studio. And then upgraded the points staying guest from the reserved studio to an empty one-bedroom.
Unless, there was an extra-ordinary situation on the
DVC/CRO end, there should never be free upgrades. Even if there was availabilty and you asked about an upgrade at check-in, normally (and technically), they would give you the upgrade AND deduct the points difference from your account.
That said, free upgrades do occasionally but rarely happen. So, if you get one, what a wonderful surprise! But don't count on it!!
Have a great time at the BCV!!! Can't wait 'til our trip!