Will the lore established in previous games be over-written by TES Online or will most of it stay relevant?

Maybe , Maybe not . It's not like any other TES tried desperetly to cling to established lore. Daggerfall changed lore from Arena, Morrowind changed lore from Daggerfall , Oblivion changed lore from Morrowind, Skyrim changed lore from Oblivion. And without doubts I can tell you that ESO will change some stuff but I doubt they will do so in such a way that would "ruin" lore

I would say in the most recent 3 games more lore was kept than lost(althoguh a fair bit was dropped as you said). and in arena/daggerfall, alot of lore wasnt even there.

as yes, arena. back when khajiit were normal elves

that said half the races weren't there

wonder TES fans that started on arena reacted when they changed khajiit from elves to cat-folk.

according to one thread in daggerfall general discussion, daggerfall vets didnt like morrowind to much .

No, Khajiit were never "normal elves", they were merely elf-like folk that were "descended from an intelligent feline race". In Daggerfall, they gained tails in addition to their still elf-like appearance. It wasn't until Redguard that they became full-on bipedal cat-men. And then they had to retcon in the Lunar Lattice to explain why they had so many different forms.

Well, almost all the lore in those games comes from people or books... both those (people and people who wrote the books) have a point of view and can lie. The same thing can be seen completely differently by another person with another point of view. I am not concerned at all personnaly.