The bracket that holds the LNBs is specific to the 121/105 SD. The KU-FSS LNB (the one that picks up 105/121) is assembled into this bracket. Plus the spacing would be wrong if you try to reverse it. The only way to get it to work would be to trade your 105 bracket for a 121 bracket.
That being said, if you do not mind having two dishes you can just use the KU-FSS LNB from the SuperDish and point it at your choice of 121 or 105. Since you have a 105 superdish, if you point it at 121 it will pick up 121, but you will not recieve 119/110. But, you could hook up 119/121 from your Dish500. A two dish solution would work.
I essentially did this since I already had dedicated dishes for 119/110, so I just use the SD for 121 (in fact I am using a SD105 to get 121).