I will tell you what will happen. Look at the price now. It has been going up very fast and much more than that of bitcoin. It is because people are actually buying the rumor. Everyone is hoping and knows that others are also going to buy the rumor, hence they all are stocking up causing the price to go up. Once the merge take place and sell the news! Those who filled their bags earlier will want to take the profit and hence sell most of the Ethereum they have been holding causing the price to go down and go then through a correction stage!