I haven't flown the MAD-BCN segment (but it's short anyway, so coach would probably be plenty tolerable). I did fly EWR-MAD however. This was Thursday outbound and Monday return. In both cases, C load appeared to be a rather pathetic 30%. In the second C cabin, there were no more than 4 people on both flights.

Based on this, you should be able to upgrade without a problem. However, you have two things working against you. First, you're flying closer to summer when loads (at least in Y) should pick up. Second, they are switching this route from a 777 to a 767 (in April). Smaller plane, fewer C seats.

One plus of the plane change is they are going to 7 days a week rather than the current 5. I still think your odds are really good, but it is still a lottery.