Is the 2003 series disregarded? .
Yes, very likely. I think - maybe - only some of the last episodes with the Coruscant battle may survive, canon-wise.
Do they interweave? Is the new one just erasing the old and rebuilding it +Ahsoka?
They never said it clear, and in the beginning it seemed they would care for the canon. But at least since season 2, TCW is clearly main canon and all the novels, comic books, games and the old clone wars series that came before are not. It is not possible to interweave them anymore, I think. To me its a pity, since I think, a lot of the old stuff was superior to TCW, and by this system, only a few things were taken over to TCW Series (like Ventress' jedi apprenticeship, for example).