Active Directory Migration from 2003 to 2012 Process Flow

we planning migrate windows server 2003 ad windows server 2012 server 6000 users,
can 1 suggest  on following .

1)what best , safe way migration

2) precautions should take,
3) how downtime take,
4) if migration failed how can revert earlier

5) how migration step step

current environment:

domain having  1 pdc(server 2003 r2) , 8 adc(server 2003 r2) in different locations

pdc having fsmo roles , global catalog

exchange server 2007 integrated active directory 

and application integrated to  active directory 

1) recommend first run test of steps in test before in production.  otherwise production becomes test.

2) doing in test, have taken large amount of risk out of upgrade since, in test should able unforseen issues.  easiest way test build virtual fence production , clone dc's , member servers want test against (this assuming running in virtual environment).  ensure production environment error free.
http://blogs.dirteam.com/blogs/paulbergson/archive/2009/01/26/troubleshooting-active-directory-issues.aspx

3) there should no downtime @ all, can extend schema , promote new 2012 dc (i recommend r2 if can).

4) before schema extension should take 2 backups on 2 different dc's.  taking 2 gives less of chance of problem if 1 of backups fails.

5)
take backup
extend schema
join the 2012 r2 servers domain
add adds role 2012 r2 member servers
promote 2012 r2 dc's
transfer fsmo roles 2012 r2 dc's (not required recommended)

if want retire 2003 dc's, need make sure clients pointing 2003 dc's dns pointing other dc's.

if retire 2003 can think updating dfl , ffl of domain , forest.

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paul bergson
mvp - directory services
mcitp: enterprise administrator
mcts, mct, mcse, mcsa, security, bs csci
2012, 2008, vista, 2003, 2000 (early achiever), nt4
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