Can one use cards like Mask change or other cards that says "HERO" in them on "Hero kid"...I mean it's a "Hero" not a "HERO"...

Based on how it would work with other languages, specifically Japanese, no. In the Japanese text of HERO cards, aside from the symbols, you might notice the English HERO. For instance, Elemental HERO Bubbleman is "Ｅ・ＨＥＲＯ バブルマン". Comparatively, Hero Kid is "ヒーロー・キッズ". Note the lack of romanised "HERO" in there. Mask change's effect's text in Japanese also contains the romanised "HERO". Therefore, with absolutely no knowledge of how to read in Japanese, we can still state that Hero Kid would not be a valid Mask Change target in Japan. Follow that through to TCG and it makes sense, as OCG card names and how those card names interact with other cards can be quite important, to the extent that cards like "Axe of Despair" always count as Archfiend cards because of the OCG name.