In my experience, people with development experience often make really good software testers. They know what frequent programming mistakes are and how to look out for them, they know how to reproduce and isolate bugs in a useful way and they are able to describe bugs in a way other developers understand them.
But whether getting moved from development to testing should be seen as a promotion, demotion or sidestep depends on the company. We can't tell you how your company values the value of testers relative to developers and that of testers. But a good hint is usually how well they pay people who have these positions.
We also don't know how you personally value the new situation. When you find that the new work is less enjoyable than the work you had before, it might feel like a demotion to you, even when it isn't meant as such. When you feel that your career is forced into a direction you don't want it to take, you should act and try to get a position which is more in your personal career interest. But that's your personal assessment of your situation.