Shortening a long vowel, lengthening a short vowel, or changing Tashdid is not governed by an automatic “always invalid” rule. Classical Hanafi texts record disagreement when such a change seriously alters the meaning.
Word substitutions are also judged more carefully: similarity of meaning, whether the substitute is Quranic wording, and the degree of semantic corruption all matter in the classical rules.
Practical lesson: correct the recitation, but do not issue a blanket invalidity ruling without identifying the exact word, exact change, and the Hanafi rule applicable to that type of mistake.