That is true, but in this case, within the time that you book, then change the date of the ticket (24 hours minimum later), the chances are small I would say for the desired flight date to be sold out (as you would make sure that the flights on the desired day would not be sold out when booking), especially as the flights is 3 months out.
But there is a risk that it does sell out yes, and the prices rarely go down on Easyjet (I have seen it only with promotions, like the recent one out of Lisboa for the anniversary of Easyjet having a hub there I think)