As you may know, the 1000.4 uses struts to stabilize the dish. I suppose that if the 500 mast diameter is the same, it could work, using the supplied struts. I think that the 1000.4 mast is somewhat longer.
The 1000.4 is larger and heaver then a 500
That said if it is in an area sheltered from the wind and propperly mounted in the first place (six lag bolts ect.) then you could re-use the mast
The .4 mast is longer to allow for wall mounting at 90 degree angels