Chuck - if you've got DTV service on their big dish, leave it alone
Realigning it after someone messed with it would be quite a task.
I was assuming eariler that you wanted to re-aim it yourself.
As for what everyone is asking about using an old 18-20 inch dish for...
IceBerg has already written that up quite well.
There's damned little worth bothering with on circular, and those dishes are too small to be used with linear LNBs.
You all probably already get NASA on an other existing system (I do on Dish) so who needs it again?
As for how to get the FTA and the Dish/DirecTV receiver to feed your TV set, I used to use my VHS video recorder for that.
It had RCA jacks in the rear, and another set in the front.
By clicking through the choices with the tape player remote, I got:
1). Satellite receiver #1
2). Satellite receiver #2
3). Local reception through the VHS player's tuner.
4). Play video tape.
As for re-using an existing mast, that might be possible.
Our advertizers have bundled deals on modestly priced dishes and LNBs.
Get one, and see if it'll fit your mast.
If not, it won't be much trouble to change out the mast while you are at it.