If you already have Outlook 2011 installed then it really is as simple as changing the server address. Work with whoever is doing the migration to test it out. I can't speak to technically about it because I didn't handle the Exchange part of the migration, but there was a server redirect that took place that simply prompted the end-user about a web server address change. Once the end user allows the web re-direct, they will continue to get emails as if nothing ever changed. But I believe something needed to occur that allowed the on-prem server to re-direct that migrated user.
Another component to consider is that Office 2011 requires a subscription service with 365. I'm not sure if you have a volume license in which case it may not matter, but if you don't then prepare to tell users how to sign in with their 365 subscription. I hope that the email address is the same as firstname.lastname@example.org because otherwise prepare to have end users put in incorrect "email addresses" when prompted to subscribe. In my experience we had email@example.com which was different than firstname.lastname@example.org. The latter is what Microsoft needed to authenticate. We also had problems with free lancers who would come and go. Apparently 365 does not like reusing the same username or perhaps its a UID. Not sure what it was, but it definitely created issues if you removed a 365 account a few months ago and tried to create a new 365 account using the same account.
Also prepare to see weird issues with how Outlook 2011 sometimes behaves when it comes to email. Essentially all your end-users will be spread across hundreds of email servers. And if Microsoft decides to make changes (and they will) then you will start seeing some users get affected and others not as it rolls out to all 365 customers. You will open support cases with Microsoft. They will ask you for logs, but because the issue is so sporadic and comes and goes it becomes virtually impossible to catch the issue as it happens. But your users will know because they will get weird prompts asking them to authenticate multiple times. Or to re-direct their web server address even though it's correct. Or Outlook will look like its connected but it just stops downloading email while OWA works fine.
Honestly, it felt like there were more quirks with Exchange Online than Exchange on-prem. Of all companies I expected to get it right, I figured it'd be Microsoft. Like most email services it works great on the web, but it says nothing about the experience in an email client. Nothing has really come close to Exchange or ActiveSync when it comes to email clients. So how can they have so many problems with it? Perhaps some of the problems I experienced were organizational-based. Now I got to deal with another beast in Google Apps. Not even going to try to use the built in OS X apps after all the issues I've heard of. Anyways, I think I've digressed enough. Good luck with your roll out.