OK, it sounds like he has a 105 SD, and he also has a Dish500 w/ 2 legacy dual LNBs and an SW21. If he uses the SD to get the 121, he will need to take the outrigger DP LNB and put it on the D500. He will also need another DP single LNB or he will need to swap what he has for a DP Twin. Sounds like heis current setup will not work. He will also need a DP+44 switch if he wants to get his locals from the 148, as well as another D300 and DP Single.
Look, If you have your locals. Call E*, tell them you have a SD for the 121, and you need the 148 installed for your locals(if you don't have them, ADD THEM). This will upgrade you to the DP+44 for free, and the installer will probably swap out your 105 head for a 121 OR swap out your legacy duals and SW21 for a DP Twin. May cost you a couple of dollars, but i would say that is your best bet.