How to coerce Lion Time Machine into offering backup history migration?
You can't get that dialog back, but you can inherit the backup manually via Terminal using the tmutil command. Do man tmutil for more information.

Example:

$ sudo tmutil inheritbackup "/Volumes/Time Machine/Backups.backupdb/iMac"


For more information, see the blue box here: http://Pondini.org/TM/B6.html and you would do well to read the whole page before starting the process.