alasdair1
Probably means they've changed the aircraft from an A319 to A320 (or vice versa) hence there's more/less rows of seats.
I have received a similar message for my flight PFO to LGW. I think they issue this message when the A320 is going to be one with the extra 6 seats. easyJet are reconfiguring their A320's from 180 seats to 186 seats by repositioning one of the toilets at the back of the aircraft and replacing the freed up space with 6 seats. All new A320's are being delivered in this new config.
I wouldn't worry too much, even though you have paid for a particular seat on the plane and easyJet have moved you, when you go to check in, you can change to any seat of your choice (in the same zone, matching the price you previously paid).
I think you can check-in online 60 days before departure, so might be worth checking-in in as early as possible and then re-choosing what seats you want, of what is left.