Transformation of Kadon Question

Here is an interesting scenario:

The spell Transformation of Kadon can only be cast by a wizard on foot. When cavalry models assault a building they dismount and become infantry. When does that dismount happen? In the movement phase when they complete their charge or in the combat phase when the assault takes place? Could a mounted wizard charge a unit in a building and then cast Transformation in the magic phase and then assault the building as a dragon?

It seems like a long shot and a very rare situation but it would be a useful trick for a Bretonnian army in the Watchtower scenario.

Fair enough, I was just trying to find a way around the "I auto-lose due to watchtower" scenario.

I've found Questing Knights are possibly the best choice for assaulting a building, they at least have S6 with their great weapons and aren't as expensive to lose as Grail knights. Here's another question: If Grail knights (or any other knights for that matter) assaulted a building, would they get parry saves? Its also not a common scenario since they'll usually have the ward save from the blessing anyway but lets say they'd lost the blessing somehow.

The spell says it "can only be cast upon the wizard, and only if he is on foot." So no reference to infantry hence my original question.

The wizard would be in a unit of knights, most likely knights of the realm who are pretty useless at assaulting a building by themselves. Using the Transformation trick would change a relatively harmless charge into something that could actually do some damage.

The discussion about questing knights was separate and a bit off-topic but currently it is pretty much the only option for effective competent building assault in an all-mounted bretonnian army.