What you're seeing is known as Revenue Management. Essentially, Disney's computers believe that between now and your trip, there will be someone wanting to book a reservation of 5 or more nights. They are basically "saving" that room for that guest by instituting a minimum-stay requirement of 5 nights.
Using minimum-stay requirements obviously benefits Disney. They earn more money because the room will be occupied for all five nights. But, it also benefits the guest that will stay in that room. If Disney wasn't "saving" it, they might have not been able to make their trip without the use of Revenue Management.
While Disney has used Revenue Management for years, they only recently started enforcing things like minimum-stays when modifying reservations. For the record, all major hotel chains have been doing that practice for decades. (The hotels where I worked back in the mid 90s enforced minimum-stay requirements on modifications way back then.)
As for being able to make a change, I concur with mom2rtk. The answer is a resounding maybe.
If you made a 5-night reservation online and immediately called to change it to only 2 nights, I'm guessing you'd be out of luck. It's pretty clear that you'd be trying to game the system.
In contrast, your situation is different as you are trying to add on to an existing reservation. As such, you are more likely to get approval...especially if your total stay (original reservation plus the two additional nights) meets the current 5-night minimum.
