Disks renamed after update to 'testing'...?

["hobie of RMN" <hobie@rumormillnews.com>]

Hi, All -

My brother has been issuing "mount /dev/sdb1" prior to backing up some
files to a second hard disk.  He lately upgraded to 'testing', and it
appears (from result of running df) that what the system now calls
/dev/sdb1 is what he has thought of as /dev/sda1, the system '/'
partition.

Thanks to the UUID= mechanism, his system still boots properly, but 'mount
/dev/sdb1' is inappropriate now, could even be the path to madness. :)

Two questions, then: (1) What caused this shift of device naming? And (2)
How do we fix it?  Is this something that can be changed in the BIOS? 
But, if so, what caused it to change in the first place?

Thanks for your time and attenton.