It really depends how you want to do it. You can get the transition pack from Microsoft that will convert your licences from SBS to single components and remove the 75 user limit of SBS, you could then move this to a new server.
Alternatively you can install a new DC and Exchange server and move your users and data across, usually quite a smooth process. However, there are some problems, as stated here the FSMO roles are restricted to running on the SBS server. If you were to use the transition pack I believe you could then move them, but if you are adding just a new DC to the domain, you won't be able to move them from the SBS server, quite a big issue.
I'm also not sure how SBS will function if you try to add another Exchange server to the Exchange organisation.
My feeling is that the best bet is to get the transition pack, you will then have what is essentially the separate components of SBS, you can then look at adding new servers, transferring roles and turning of the SBS server.