To the best of my knowledge:
The only part of the new services that requires a new radio is a la carte channel selection.
If you currently have XM, roughly 10 channels of "Best of Sirius" will be added to the XM platform, and you can choose to pay what you pay now to keep roughly the same XM you have now (subject to some amount of duplicate service consolidation - only one 80s channel, etc) - OR you can choose to pay more to also get the "Best of Sirius" package along with your XM subscription.
Similarly, 10 channels or so of "Best of XM" will be added to the Sirius platform, and will be available to Sirius subscribers for an extra fee. Unfortunately, unless Sirius makes deep cuts in its existing service, older Sirius receivers may never be able to receive "Best of XM" due to the apparent 128 channel limit. (XM is fine because all receivers can handle at least 256 channels AFAIK.)
Depending on what they decide the "Best Of Sirius" actually is, I might be willing to drop my Sirius sub and just add "Best of Sirius" to my XM radio.
Also, it's not at all clear whether Howard or NFL would be included, or any other big name talent, because Sirius, as the new parent company, would have to renegotiate for permission to send those channels to XM customers. Some people seem to think that they will be able to pick any 10 channels that they think are the best of Sirius and add them to XM, or vice versa, but this will not be possible until they come out with the new a la carte compatible radios. Existing radios will only be able to receive (for an additional fee) a subset of channels from the opposing service that are selected by company management.