The crowds will be huge--on a scale of 1-10, figure an 8. Could be worse, but you will need to follow all the tricks--eg avoid the parks with EMH morning hours, go to other parks instead those days; show up at turnstiles at least 30 minutes early, make dining reservations so you don't have to wait in line forever, and try to see the fireworks at MK and Illuminations at Epcot on nights that they have night EMH so you aren't trapped in a massive crowd exiting.
If you are night owls, the later parts of the night EMH can be a fun adventure...