First of all, ones opinion about Lubavitch isn’t s litmus test of their credentials as a gadol, because it’s very likely that many gedolim simply did not know what was going on in Lubavitch; they held of what they thought Lubavitch was. This happens sometimes.
Secondly, rav miller stopped mentioning the Lubavitcher rebbe as a gadol in his later years, precisely for this reason – more information about neo chabad became known. Rav pam was the same way; for years he said thet rav shach was talking to bnei eretz yisroel and that we shouldn’t be part of it, but in his later years was chozer.
It’s not that they did teshuva, it’s that more information became available and known. This was before the internet; information traveled slowly. The truth about rabbi kook also didn’t come out for decades, with many gedolim defending him simply because they did not know the extent of his nationalistic heretical statements and ideas, such as the frei jews having higher neshomos than the frum, because they build up the land etc..