The cost difference between a 3 spd and 6 spd is less than $100 for a new bike. Contact a shop and see what they would charge to upgrade your 3 to a 6 and figure anything over $100 is $ that would make up part of the difference between what you would sell your bike for and a new one. Some people don't want to wait 2 months for a new one and would like to save some $ on a used one. Maybe you should research used prices and see what you might expect to sell yours for. It may make moving on to a new one reasonable or perhaps the cost difference would stll be more than you are willing to go. Worth looking into.