It was us that changed it by adding the "u". Americans still use the old English spelling without the "u".
Indeed. There are loads of examples of what we think of as Americanisms which are in fact old English; the term 'Fall' for Autumn being another example, as is the verb 'to skedaddle' and even 'bite the dust', which I thought came from the Wild West. And it's still happening today, witness the number of people who think 'soccer' is an American term.
But back on topic it has been mentioned but I do love the way "just going for a fag" means two totally different things! (Or maybe even three!)