No problem.
OK .. On the backup, the answer is no.
The backup can only be restored to the same version.
That's a good lead to the BIT.
The BCM Imaging Tool allows you to reimage a BCM HDD to any of the images available.
When you purchase the BIT, you get a s/w CD.
You need a CD ROM that runs via IDE or USB.
You also need to appropriate image to apply to the BCM HDD.
So, you really only need the BIT if the upgrade fails and you want to get back to where you started.
The 3.5 to 3.6 will take a couple of hours.
The 3.6 to 4.0 is about 1 hour 45 minutes.
Once you get to 4.0 apply the latest SU patches at a minimum. I suggest applying all of the current GA patches.
I've taken to not even doing a backup prior to the upgrades. We really aren't equipped to use the backup if there is a failure and it just takes up extra time.
If you really want to be prepared, get a 3.6 HDD and a 4.0 HDD in case of a disaster. Or, you can buy the BIT and get the images for 3.5, 3.6, and 4.0.
Good Luck!
-SD-