Hi Scott,
First of all, great questions!
We haven't started this process with our customers but we have started the process internally so I can share how we are doing this.
1. Since we are involved in the Microsoft TAP Program and received access to a Beta version of Microsoft Dynamics CRM 2013, we are preparing a Lunch and Learn session to navigate through all the changes and see what's new between CRM 2011 and 2013.
2. On the weeks prior to the Lunch and Learn session, we have been sharing videos and posts by Microsoft regarding the new features of CRM 2013 and ask the users for feedback to make sure they have read/scanned through the information, that way when they get to the Lunch and Learn session they won't be lost (I.E. There is a new version coming?!?)
3. In regards to training and testing: When we received access to the Beta version of CRM 2013, we tested all of our solutions to determine if something will break, fortunately we haven't found any issues (awesome work Microsoft!) and everything translated flawlessly. Unless you have old (a.k.a. CRM 4) code, your configuration and customizations should translate perfectly. At least that's what I've seen on my side.
The new interface is surprisingly easy to get used to, everything is in the same place (I.E. Sales, Marketing, Service, Settings) so there won't be a learning curve to know where "all these sections are now located". Users will need to learn more about how to navigate the new interface (I.E. No Ribbon) but the way they got to wherever they were going will remain the same. :)
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Now, I understand not every company or partner will have access to the Beta version of CRM 2013 (and this is why we haven't started this process with all of our customers) but in the meantime I would focus on reviewing the plethora of information that has been released by Microsoft. There are not only awesome blog posts describing the features but videos as well where you can see the product in action.
One last piece of advice, if you find information out there (at least on the next upcoming days) that seems new or not released by Microsoft or someone related to Microsoft, take it 'with a grain of salt'. I say this because all the participants of the TAP program (I.E. the people who have access to the Beta and know the product the best) are under strict NDA Rules with Microsoft and have not released or published anything about CRM 2013 without the consent of Microsoft. This means that what you see out there has been released by Microsoft first (perhaps covered in more detail by an MVP or some other TAP Participant on a separate post) or by someone who is speculating at this moment, so be careful about what you read.
I have had conversations with customers who say things like: "I'm excited because I read somewhere that CRM 2013 will include a client for Outlook in the MAC!" or something along those lines which in some cases can be completely wrong or simply not accurate.
Good Luck Scott!