Webbydude,
NO NO NO, the 322 will work just fine with both legacy LNBF's and switches as well as Dish Pro (and Dish Pro Plus) LNBF's and switches. The 3900 will work with Legacy LNBF's and switches and DISH PRO PLUS but NOT Dish PRO (NON PLUS) LNBF's and switches (without a Dish Pro adaptor).
If the OP has a Legacy Switch either (SW44 or SW64) then he doesn't need to change anything although it will require two Cable feeds from the switch to connect the 322. If the OP has a Legacy TWIN then he will need to either replace the Legacy TWIN with Two Dual Legacy LNBF's with a four output legacy switch (SW44 or SW64) or get a Dish Pro PLUS TWIN (although if you have poor or low grade cable it will likely NOT work).
If the OP doesn't want to run extra cable runs or doesn't have a SW44 or SW64 then the Dish Pro PLUS TWIN would be a nice option as it would only require a Single cable to each receiver (Although the 322 would require a Dish Pro PLUS Separator). Another thing to consider is the quality of the existing cable. NON RG6 or poor quality RG6 may NOT work with a Dish PRO PLUS LNBF.
John