Confusion of sounds in a language

Are there instances of a set of speakers of a language, over a period of time, swapping the pronunciations of two different phonemes in their language?

For example, speakers of Ceceo Spanish merged the /s/ phoneme into the /θ/ phoneme, while speakers of Seseo Spanish merged the /θ/ phoneme into the /s/ phoneme. Sometimes speakers of one or the other who attempt to “correct” their speech due to sociolinguistic pressure may end up pronouncing each of s and z as both /s/ and /θ/.

I was wondering if there are other examples of languages in which the characters representing two phonemes are “confused” in this way, or, even better, if there have been any cases in which the swap is complete (e.g., were Spanish speakers to always pronounce s as /θ/ and z as /s/).