The vowel sounds in “près” and “pré” are very clearly different, and the sound in “prêt” changes from “è” to “é” when in liaison because it always sounds like a long “è” at the end of words (and separately, in closed syllables) and always sounds like “é” in open syllables otherwise (liaison triggers a change in the syllable structure which changes the vowel here). This does not contradict what I said. You said “pré” and “prè” sound the same, nothing about “prê”.