"Not the brain. The mind," Sirius says. "All physical changes to the body have to be made."
"But from what you say, it sounds as though you achieved the correct outcome," Remus cuts in. "And in just a few weeks, too."
"That's true," Sirius concedes with a smile. "And that suggests you'll be able to manage the physical aspect of the transformation, or pick it up very quickly."
...which leaves the mental aspect.
"As for the mental aspect..." Sirius starts, thinking about how best to say this. "I told you the last time we spoke that are other ways of determining whether you're still... afflicted or not. It's not physically invasive at all,# but it is time consuming## and requires specialised equipment. It practice, it breaks down to something like taking photographs of you, over a period of time, from which it's possible to make reasonably* accurate** inferences about the condition of your soul."
#Sirius has told Q earlier that it is emotionally, rather than physically, invasive. ## And therefore expensive, unless the diagnostician is doing it for his own interest, and not charging by the hour six-minute-increment. * For a given value of 'reasonable'. ** And a given value of 'accurate'.