The reward change for miners is 80%, 50 to 10 Gulden. This is where most of the new Gulden goes on bittrex, Witness change is 50%, 30 to 15. Witnesses, most will keep there coins and use it to increase the witness amount with compound interest feature.
Dev reward only drops 18% but all those coins are sold on blockhut. Blockhut will have less coins to sell after 1st July.
All the rewards will drop 50% every 4 years after this change.
It will take 2 or 3 months after the reward change for the market to adjust to the new reality. You will notice with bitcoin that the price doesn't jump straight after the reward halving but takes 3 - 6 months. Traders will try to collect as much of the pre halving rate value as possible, until the upward pressure can no longer be denied. My prediction is 2-3 months of these prices with range of 50-75% in dips and jumps.