First of all, lounge access is guaranteed for Diamonds. You don't need to research Flyertalk wikis for that. It's only Gold status that is hit-or-miss.

As for date adjustments, you essentially need to cancel your existing reservation and rebook using currently available inventory. This is why you should consider making multiple reservations when you aren't completely sure of your plans. (5 night blocks, etc.)

I personally would switch over to the Indigo for the last two nights. It is much more centrally located than the Hilton Diagonal Mar, and is far more convenient for the bus to the airport from Plaza Catalunya. Certainly better than paying a huge number of points for a "premium room" and having to switch rooms mid-stay. (or stay in the standard room whilst paying for a premium room)