Is it safer to carry on undergraduate research areas to gradschool or start from scratch in an unknown area?
If you start by thinking about what's safe as an undergrad/grad, I think you may experience (unpleasant) surprises down the road. Do what you like best! Grad school is not an easy path, and motivation is sometimes hard to maintain throughout, so the best way to help your chances of success is actually to do something that you enjoy (and hopefully will enjoy for the next few years).