I stopped wearing my retainer 6 months ago is there still a chance my teeth will move?

I wore my retainer for 6 months after receiving braces however i lost the bottom set and then stopped wearing the top as i didnt deem it important, the top set of teeth havent really moved however i can no longer fit my retainer on and im worried my teeth may move more. The top set doesnt appear to have moved alot in 6 months since however im worried they will?