When in the live session you should be able to just choose 'something else' from the installer options, choose to install the OS itself to 'root' or / (put a check in the box to format it ) and leave swap alone as that won't need formating, leave your home partition alone too/ ensure you DO NOT put a tick in the box . . . . . . This is the method I use all the time to upgrade, all your home files are safe and you're re-installed in no time.
Edit: having not used a debian DE for some time I'm not sure if any desktop tweaks will still be good as you're moving from a Deb edition to an Ubuntu based one, hopefully someone else may chip in and clarify.
Lenovo ThinkPad T61 LM16-64 bit Intel T7500/2.2GHz/Cinammon 1.8 Intel GM965. Toshiba Satellite M70: LM16-32bit. Desktop:LM13 Maya 64 bit, on new Intel 3.2ghz proc/asus MB/8gb RAM