Depends; the 18-70 is one of the best kit lenses produced in the last few years (I still use mine, a LOT), but it's still a kit lens. If you can 'trade even', that is get enough money from the sale of your 18-55 to cover the 18-70, I'd say go for it, but if it's going to cost you additional, I'm not sure that you wouldn't be better off putting that cost toward a new/different/better lens.