That depends on your reasons for wanting to start over.
Personally i have done a few 2003 to 2008 exchange migrations and only had one fail but that ws a dodgy set up to start with.
I have found the migration to be relatively reliable and less work than doing it all from scratch also the migration is really only the domain settings exchange and all the configuration. So it does not bring over pre installed programs so you will have to reconfigure things like blackberry server separately.
The migrating works buy modifying the registry for I think 21 days and allows a second PDC on to the network, were it then replicates all the data as if you had a standard server and a couple of PDC replicating across the domain. once it had replicated all the data it then kills the old SBS effectively demoting it from PDC. and then activated the new SBS08 as the only PDC allowed in the SBS domain.
I would strong recommend using the migration as it is how ms intended it to work has good wizards and there is a full document on how to do it. and some extra info and some watch outs Note please be sure to read the whole document before starting as I found some things are at the end of the document that need to be done at the beginning. Like make sure the exchange limits on the new domain are larger than the largest mailbox on the old domain els it screws up
most importantly have a full backUP id recommend shadow protect evan the trial will do perfectly for this purpose just install the sbs version and use a large usb hard drive to take a full back up this is an awesome bit of software that will give you a month trial back up to an image and you can always restore the server to before the migration and nothing will have been broken.
little note make sure both sbs servers are fully updates with all patches before starting and again reread the full instruction before stating as i found some things to be in the wrong place .
The beauty of the migration is that once you are done everything pretty much works not need to rejoin pc's set up account importing mail all just works also note it can take hours to migrate like the exchange part i leave over night and look out for the settings that just skips mailboxes it cant migrate as you can always do them latter or manually .
extra reading
the migration document
http://www.microsoft.com/downloads/details.aspx?FamilyID=52b7ea63-78af-4a96-811e-284f5c1de13b&DisplayLang=enextra watch out for stuff
http://technet.microsoft.com/en-us/library/sbs-2008-2003-migration(WS.10).aspxlet me know how it goes feel free to ask any questions
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