You can modify every aspect of the campaign while it is live. However, there are some rules you should follow.You can modify the prize-winning probabilities during the campaign the same way you set them up. After saving all the changes make sure to click “Update live campaign” to apply the changes to the live version.
Although you can do that, be aware that you should communicate this in your campaign terms and conditions under fair play. When you are increasing the probabilities towards the end of the campaign then the players who arrive later would have a higher chance of winning the prize which creates an unfair campaign environment.