A system state backup of your Schema master domain controller should give you decent options. This assumes that you aren't going to change anything in AD or Exchange with the exception of stuff that needs to be changed for the migration. I'm assuming this is a small environment. If it fails, restore your system state back to that DC. Also, if you can take any other DCs offline for the initial migration process, that'd probably be better as well (as long as your schema master is also a global catalog server). You should be able to tackle your major changes in the first day.
You're going to have to migrate eventually though. You might as well fix whatever problems you have now. I'm very doubtful that Microsoft will support 2003 migrations after 2010. If you just get it over with now, you'll have a few years before you'll have to worry about another forced migration.
Since you have a few DC's, I recommend just taking the Schema master off the network for the schema upgrade. If your schema master is on the 2003 server, move the role to a 2008 server, unplug it from the network, take a system state backup, do the schema upgrade. If that succeeds, plug it back in and let the other DCs replicate. All of your changes to AD after this can be reversed as necessary.
The Schema master needs to be moved to a 2008 DC because the Exchange installler will not run on 2003. If you are going to isolate the Schema master for the purpose of ensuring the schema updates properly, you need to have that role on a server you can run the schema update from.
The reason I'm recommending that you isolate the Schema master is because this will stop a failed schema update from replicating to the other domain controllers. I've never seen a schema update fail, but if it did and the DC is isolated you can restore your system state and not worry about the faulty schema replicating. Once the schema is changed, it cannot be unchanged. If you don't have it isolated and it replicates a bad schema there's no way to undo that.
The schema update process does not need access to all DCs, only the Schema master. After the schema updates properly, plug the schema master back into the network to replicate. You then have access to all DCs (although I don't think you need access to all DCs in order to install Exchange. Just Schema master and a Global Catalog).