My view is that 'heavy metal' changed, not the bands.
The likes of Zeppelin, Purple, Heep and even Sabbath always had a hefty dose of music as far away from modern 'heavy metal' as Lena Zavaroni... but a fair chunk of their songs were foundation on which heavy metal was built - Whole Lotta Love, Smoke on the Water, to me, are unquestionably so.
Ironically, some contemporary jazz (Soft Machine) and full-on prog (e.g. High Tide) was vastly 'heavier' than any of those four bands until Sabbath Bloody Sabbath came out (IMHO) but i think these things are intensly personal and everyone will have their own views, which is what makes it interesting.
Tell me this isn't prog AND heavy... 🙂