Removing exchange 2003 organization from 2003 Domain

Hello,

My company has fully migrated to Office365 (NOT Hybrid environment) from a local environment consisting of a single domain with a single exchange 2003 server.

Now I will be upgrading the domain to 2012, but I plan on removing the 2003 exchange organization first, before I upgrade domain to 2012.

Would this be the best approach?  Will this remove any needed the exchange attributes?

Thank you