Self-published authors have to work hard to build up a fan base and to establish themselves as "real" authors in the eyes of their readers. That's why stepping outside your genre and writing something completely different can be pretty tricky and scary business. What if you go out on a limb...and lose all of your fans?
Outside the Box
After putting all that time and effort into building up a fan base, stepping outside that comfort zone with a totally different book is a brave thing to do (some might say foolish). While some of your fans may stay true, others may be turned off because they aren't fans of that particular genre. That means you've got to start all over again, and start targeting fans in your new genre to find the readers that will be interested in this new book of yours.
It's a lot of work, but it's not all that different from all the marketing you've already done. You should re-focus your efforts with every new book you release, whether it's in the same genre as your others or an altogether different one. Stepping outside your genre actually gives you a unique opportunity to gain an even bigger fan following, and reach out to readers you mightn't find otherwise.
Don't ever be afraid to make a change. One of the joys of self-publishing is that you get to do whatever you want, write whatever interests you, and let your own skills as an author develop and grow in any way you like. You don't have to answer to anyone, uphold contracts or write sequels you aren't really feeling. If you have to do some extra marketing to get more readers, that's just something that comes with the job.