Sure. People move topics around over the course of a career. However if you are going to make a radical change, it will take a lot of work — you will need to make sure you know the literature, debates, evidence, modes of thinking, languages, etc., that are relevant to the other topic. Retooling across vast time spans or countries is a difficult thing — if I wanted to become a medievalist, I'd need to essentially do all of the reading and work that a new grad student would need to do to become a medievalist, if I had real intentions of making scholarly contributions. In practice this is a pretty rare thing as far as I can tell. More common are smaller branches, e.g., I work on the 20th century, and I could expand that into the 19th. Or an early modernist might shift to the late medieval period (or vice versa).

But is it possible? Sure. There aren't any history police, thank god.