You can choose the partition you want. When the iso boot is in the format screen, select the "options" button and then you can select the partition.
For the others - Once MS is in Escrow and RC branches, they normally disable the ability to do upgrade installs due to possible internal changes that are not done in upgrade installs. I was unable to do an upgrade from 7068 to 7077.
This Can be bypassed but I don't recommend it, here's the way for anyone tempted to try it
1. Download the ISO as you did previously and burn the ISO to a DVD.
2. Copy the whole image to a storage location you wish to run the upgrade from (a bootable flash drive or a directory on any partition on the machine running the pre-release build).
3. Browse to the sources directory.
4. Open the file cversion.ini in a text editor like Notepad.
5. Modify the MinClient build number to a value lower than the down-level build. For example, change 7100 to 7000
6. Save the file in place with the same name.
7. Run setup like you would normally from this modified copy of the image and the version check will be bypassed.
Some iso's available out there have already had this applied beforehand so it may upgrade successfully, but I don't trust pre-modified builds as you never know what else is changed, and I only get builds where the hash check verifies it.