Most of the “changes” are the data. Data is stored on external storage. I don’t care about updates and patches (I take notes about customization), the worse they can be dangerous. So, almost anything regarding full (incremental or not) backup is not related to Qubes, except the single full backup instance.
I have backup of my data, but created on external storage too, and within qubes, not created by Qubes. Also, I graduate my data in terms of a backup. If something is extremely sensitive, I immediately do create several copies of it on several places at once at the moment of manipulation. On the other hand there are other data I can live without, so I back them up once in a several months (from one external storage to the other, within a qube).
I’m too old to live in a 'what-if" fear. Instead, I know what to do “when” it happens and prepared myself to lose some things. They’re just bunch of “0” and “1” at the end of the day, and I will not bring them with me to the grave.
For example, I lost my Win qube trying to import it from backup after upgrading from 4.1 to 4.2. It doesn’t work under 4.2 with Xen PV disk drivers for Win. So what was the worth of that backup? That’s what I meant when I wrote “will I be able to restore it?”
Well. my strategy was that all the data actually were on the external storage, independent of the qube itself. So, since we cannot attach block devices any more, I had a “clean” win qube from a single instance full backup, without qwt installed, I restored it (I could create a new one from the scratch, it’s faster, right?), found a workaround to attach external storage, and voila.