I know it's a sound change, but is there a specific name for it? It's for an assignment I'm writing on the phonological transparency of the suffix -ity.
This question is a bit complicated. -ity is not really a productive suffix in English; it is the English outcome of the Latin suffix -itas. In the transition from Latin to Romance the sound represented by the letter “c”, if followed by a high front vowel, becomes first /ts/ and then, in French and in the French and Latin loan words in English, /s/. So there is not a direct shift from /k/ to /s/, but a staged shift of /k/ > /ts/ > /s/. This can be called affrication followed by de-affrication.
A quite generic term for this phenomenon is Alternation and since it involves consonants, you can also say consonant alternation.