I'd suggest checking with the bike shop again. Did "adding one cog" mean going from 9 speed to 10 speed or from 10 speed to 11 speed? More to the point, was he talking about adding more gears (which you don't need) or switching to lower gears (which you do need)? There would generally be a possibility of going smaller on the front rings and larger on the rear rings, with possibly a longer derailleur required on the rear.