The option would be custom install rather then upgrade. The upgrade compresses the previously installation where you see the rollback option upto a point there. The custom install however packs everything up into the Windows.old folder that started with Vista.

If you had asked the MS rep about a Vista upgrade disk I can assure you that you would have most likely heard the exact same thing about the need for a previous version's disk like ME, 2000, XP for the same reasons.

But as I was saying before We'll just have to wait until someone actually has an upgrade for 7 to look at first before that can be fully answered.