The SV650N has a 15 tooth front & a 45 tooth rear... The SV650S has a 15 tooth front & a 44 tooth rear - Yes you
can change the ratio by just changing the rear sprocket, in fact, just changing the rear is
probably better than reducing the size of the front sprocket as this increases wear on the chain quite substantially...
The sprocket sizes increases in diameter with the increasing number of teeth added, this is why you need a chain with more links in it to compensate for the extra size of the sprockets... (e.g. I want to put a 47 tooth sprocket on the rear of my bike, to do this I need a 110 link chain, as opposed to the 108 link standard chain I have fitted at the moment...)
TBH I dunno whether this answers your point or not, this is just my 2 peneth
