Lukc is correct. Are you into Linguistics, or conlanging? If not and you want to reproduce that effect still, I can link you to some amazing sites that's easy to understand. I would suggest learning what IPA is, phones&phonemes, allophones and some other morphology terms first etc etc before going into the rest.

It reminds me of Sound Change, but (I am still new to it of course) the example you gave is two separate languages, so I am unsure if "Sound change" is the correct term I'm looking for....hmm.