[Samba] Migrate DC from Small Business Server 2011 to Samba 4

[Thor]

Hi,

I want to migrate a Windows Small Business Server 2011 domain controller 
to Samba 4. The SBS2011 has about 40 users and 100 client computers. 
Also about 50 GPOs are in use. Furthermore there is Exchange running on 
the SBS and it has to stay there for the next couple of month (it will 
be replaced by a new mail system later).

So far I think I can either try to migrate or start from scratch.

I guess migration can be done like this (and probably much more steps):
* install Samba
* join Samba to the SBS domain
* sync SYSVOL folder from SBS to Samba
* move FSMO roles from SBS to Samba (is this even possible with SBS?)
* actually then shutdown SBS, but wait... It needs to keep running for 
Exchange

Or should I start from scratch like this:
* keep the SBS running with old domain (mydomain.local)
* keep Exchange on the SBS. Should still be possible to connect with 
Outlook.
* install Samba
* set up a new domain: mydomain.com
* create all users with new passwords by a script on Samba server
* connect all computers to the new domain (do I have to do this manually 
or is there a better way?)
* move the GPOs to new DC (how?)

There are probably 1000 things more that I also should think about...

Thank you,
Thor