Except maybe the 921, all older receivers should work with the X-Ku LNBF to receive all the programming they are capable of receiving.
You can get an LNBF and change it yourself, but you need to get the V-code changed to the X-Ku one.
The way the X-Ku LNBF works is it tunes the normal Ku band 680Mhz or so higher. The local oscillator in the LNBF is 10.070 Ghz, instead of the standard 10.750 Ghz. With that, all legacy receivers can technically "see" the new satellite, but chances are what will go on the new satellite will be MPEG4 only.
I have a 421C, 315, 505, and 605. Last I used the 421C, it was set up for X-Ku, and worked fine. The others all work fine. A tech changed the LNBF and the V-codes for the receivers.