I was running Mac OS X Yosemite prior to installing the developer preview of El Capitan. After successfully completing the upgrade, I started up Apple's Mail.app application and it went through the process of upgrading my existing mailbox (which is about 22GB in size). The upgrade did not complete successfully.
Message Import Fails on Upgrade From Yosemite
Same here. I am stuck for 24 hours on the Mail Upgrade window.
https://www.dropbox.com/s/ab6h4z3lsb0gfgs/Screenshot%202015-06-27%2020.51.30.png?dl=0
I am having this problem in the El Capitan GM 1 - OS X 10.11 (15A282a). It imports approximately 8000 of my 203.3K messages, then the counter stops incrementing, but the time remaining starts and continues to increase until I cancel.