EV7 to EV10 Upgrade Path

I am starting a project that will upgrade Exchange from 2003 to 2010 SP3

About 7,000 mailboxes in total.

The Customer has a mix of Outlook 2003 and 2010 clients

They have EV7 with about 1.3TB of archived mail.

I am looking for the upgrade path for the EV environemnt to get to EV10 with the least possible downtme.

So far searching the site has not got me what i am looking for and my guess is that this is not a 20 minute hit enter and wait thing so it would be nice if there was a roadmap that may point me in the right direction as well as the best upgrade strategy and processes.

Thanks in advance for the help.