Dont think the league will allow them to do that- move grounds for certain games. Thought it was one ground only May be wrong though
If there are fears that a ground is not fit to hold a certain game they are more than capable of moving it to another ground providing that the ground they are moving the fixture to is available and the owners of that ground allow it.
Would also need consultation with the police and FL obviously.
i think you will find that, whilst they can do this, they can't do it to cash in on a big gate - the maximum number of tickets available would be whatever the capacity in the normal home stadium is. the fa changed the laws a couple of years ago after farnborough moved their home fa cup tie v arsenal to highbury to make the most of the gate. as i remember, the chairman left shortly afterwards with all the cash.