Airlines have a summer schedule and a winter schedule, and there are two set week-ends every year when they change from one to the other. The next "all change" week-end is in a couple of weeks.

The next season's schedules aren't usually finalised until a month or two before the schedule change week-end, so if - say - you book a flight in February for travel in April then there is a significant risk that the time of the flight will change. Just one of those things, and it happens with all airlines, not just Easyjet.

Most of the time the changes are minor, but sometimes you get major changes like this one. I've had a few major ones, like AF trying to move me from a Saturday morning flight to a Friday evening one because the Saturday morning one no longer existed (I managed to find an alternative which was slightly less inconvenient, and AF were happy to accept my alternative proposal - but of course it's a lot easier for them as they are a legacy carrier who can book passengers onto other legacy carriers)

You will need to look at the fine print, but somehow I don't think you'd get anywhere by taking them to court. What happened to you, while most annoying, is normal and commonplace, it happens to thousands of passengers every year, I'm sure that if there was a chance of winning court cases over timetable changes then those court cases would have been won years ago.