Microsoft Certificate Authority (CA) PKI Upgrade

Current Configuration:  Win 2008 R2 Domain, Win Server 2008 Enterprise Offline Root CA, Win Server 2008 Enterprise Subordinate CA.

I'd like to upgrade our CA environment from 2008 to 2012 R2.  I was thinking the best way is to create a parallel environment with a new Win 2012 R2 Std Offline Root CA and Win 2012 R2 Std Subordinate CA.  So both CAs would be active at the same time.   Old_RootCA_2008 > Old_SubCA_2008  and New_RootCA_2012R2 > New_SubCA_2012R2.   Then we issue new certs from the new CA and once the old certs are no longer required we decommission the Old CAs.

Is this a reasonable approach?  
Or should I keep the Old_RootCA_2008 and just create a 2nd Sub CA New_SubCA_2012R2 and decommission the Old_SubCA_2008 ?  
Or should I work on upgrading the 2008 to 2012R2 and how would I do that?
Any other ideas or advice?

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Natty GregIn Theory (IT)Commented:
The first instance makes more sense, and gives you options for troubleshooting should something go wrong. My 2 cents

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DigitalInfuzionAuthor Commented:
Thanks for the feedback.
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