It's an interesting approach; I wonder if the powers that be are aware of how much work and hassle will be involved in this. Then again, depending on how badly broken it is, and the size of your user base (which I gather from your previous post is rather small) it may well be the right way. If it was me I would definitely be looking to exhaust all possible alternatives before definitely committing to the new build, however.
I gather from your previous post that you have a multi-forest environment, so an alternative option would be to just migrate the users, computers, services, etc from the other forests back into a single central domain. Even just dropping and setting these up from scratch would be a hell of a lot less hassle than recreating everything from scratch.
Then again, as you said, the powers that be have made the decision, so there might not be anything at all you can do to change this.
For Exchange the only really sensible migration method for you is to use exmerge to dump out your mailboxes to PSTs, then import them back in, either via Outlook import or exmerge again. Be aware however that you'll lose single-instance storage by doing this, but also be aware that losing single-instance storage may not be that big a deal.
There are also limits with exmerge; I haven't seen more recent versions but the last time I used it (Ex 2003) it didn't support Unicode PST exports, meaning that there is a 2GB PST size limit. Other practical considerations such as loss of SIS and general PST overhead may mean that for any mailbox over 1 GB you'll need to export from Outlook.