I do not believe it is a big deal for Pope to wash a girl's feet. He can do what he likes and change whatever Law he wishes, indeed it will be seen as a very popular move by the media and the majority of Catholics, who fail to understand this action as a sign of Christ washing his Apostles feet.
It is however a very big deal for the the Supreme Legislator of the Church to break the Church's law or to set those laws at nought, especially as Supreme Legislator he has the right to change the Law. It does not bode well for a "Reforming" Pope to do it.
I don't want to join the doom and gloom merchants but if the clergy of the Church, as well as the Pope, can choose do without its Laws then either we have become perfect society or we indeed have society of robbers and internal cohesion of the Church is broken and where that happens it becomes a place where brotherly love turns into mere Relativism and we have returned to the age Borgias.
Then I will be free to marry, or to marry same sex couples, admit those living in adulterous unions or even admit non-Catholics, pagans and pet dogs to Holy Communion, refuse to hear Confessions. I could even if so felt inclined prostrate instead of genuflect after the Consecration, and if I was a bishop ordain women to the diaconate or even the presbyterate or episcopacy, if were a lay person, an abortionist for example, why shouldn't I go to Holy Commion?
Keeping the law is a mark of Communion. Without the Law anything is possible, no-one has protection or rights, the guards are unguarded and no-one can be trusted!
This little scene from A Man for All Seasons is perhaps apposite.