DAR

While the vast majority of users have been migrated with no problems there is no guarantee.  If your data is important to you, it should be backed up whether you are been migrated or not.

Microsoft clearly states in the terms of use, that you are responsible for the data not them.

I tell everyone who stores data in the cloud to back up their data or risk losing it.

1-You probably wont lose your data
2-If all goes well you will be able to use them both before and after the switch.


 

You should regularly backup the content that you store on the cloud. Having a regular backup plan and following it can help you prevent loss of your content.
One of the biggest issues with web based emails is that if you are ever hacked and dont have a back up you have lost everything.

http://windows.microsoft.com/en-US/windows-live/mail-import-backup-restore

The best way I know of to backup your email is to install Windows live mail, configure it, and sync it.  Seems like a bit of a PITA but if your data is important to you worth the effort.

http://ask-leo.com/how_do_i_backup_windows_live_hotmail.html
http://go.ask-leo.com/windowslivemail (for WLM 2009)

http://www.emailquestions.com/windows-live-mail/2487-backup-windows-live-mail-2011-a.html (for WLM 2011)

Remember Microsoft is not responsible for securing your data, you are.

Don't forget to back up your entire computer as well.