I wanted my (test) BE to also move to Jessie but didn't want to go through the long process of an upgrade from BE (Wheezy) to Jessie.
I recommend waiting until Jessie is stable and the BE to Jessie upgrade is fully documented and well tested.
With that said, this is how I did my BE to Jessie upgrade. (I like living on the edge sometimes and usually know how to fix things when they go wrong),
Caution, not for the faint of heart, I always have a live USB handy and know how to restore and update grub2.
(If you don't know exactly what you're doing, don't EVEN try this!) I have full backups of all partitions.
1) I coppied my (already upgraded to Jessie) HE-root partition using gparted then relabeled the copy to JessieBEroot.
2( Next I changed the UUID of JessieBEroot using tune2fs.
3) Then I used blkid to list all partitions so I could revise fstab file in JessieBEroot.(Both Root and Home partition UUID's needed to be changed to those of JessieBEroot partition and my existing (test) BEhome partition.
4) I also had to revise the UUID number in JessieBEroot grub.cfg so it would boot. (Not recommended changing this file but it's the only way I know to boot a partiton where the UUID number has been changed).
5) Next I booted to liveUSB and reinstalled grub2 to my favorite install then booted the HDD to that install and updated grub2.
Note: User name was the same for my original HE install as for my BE install or I would have needed to change /home/user of BE Home Partition to that of my original HE.
6) Next I booted into JessieBEroot and did an update-grub from there to make sure my grub.cfg was revised.
The process worked fine but there's a lot of potential to really mess things up so don't try this unless you really know what you're doing, have full backups of everything and are willing to pull your hair out if it doesn't work for you.
Doing this didn't take near as long as a normal upgrade from Wheezy to Jessie but it requires knowing the steps and what to do ... not if/ but when ... things go very wrong.

Dave