[harsh]

I am moving in about a month, and I am wondering if I should go with the 5 LNB dish now.

Now as in tomorrow or now as in "about a month"? The 5LNB dish isn't going to buy you anything today.

I understand that the new dish must be installed on top of the roof, on a pole, or to masonary?

Very few mounting configurations are being dismissed out-of-hand. Chimney straps may be the lone exception

I am planning to get a new OTA antanna, a SS-2000 square shooter that could be installed right on the 3 LNB dish. Could it be installed on the 5 LNB dish, too?

NO! The Square Shooter mounts on its own mast in much the same way that a satellite dish is mounted. Because the new multiswitches do not handle OTA, proximity to the dish or multiswitch is typically not a consideration.

I am planning on moving all my equipment, including my 5x8 multiswitch. I believe the entire setup is going to work, even diplexing the OTA signal since I don't have a receiver capable of getting local OTA stations.

You would be wrong about the 5x8 switch working with the 5LNB dish. Diplexing must be done downstream of the 6x8 multiswitch.

You will need to migrate to the 5LNB setup eventually, so you might as well do it as part of the move. The question that remains is what exactly is this "another HD Reciever".