Whether you can easily switch to a wider range cassette depends on your rear derailleur. They come in different cage length to accommodate different amounts of chain wrap. Find out your derailleur model, then look up the specs on it. You'll see there is a largest cog it can take and a maximum cog difference. If you can change to a wider range cassette within those specs, then go for it. You made need a few more chain links too, but your LBS can do that easily. If you want to go beyond those specs, then you need a new derailleur as well as cassette (and probably chain).