I've been searching on every columbo site, but can't find an answer to this: it looks like rudy, the uncle victim, changed the will only around 10 days before being murdered, according to what the lawyer says, it seems he had intended to leave the paintings to Dale, instead all of a sudden he decided to leave them to Edna.

I know columbo and edna later have a discussion about this, but I'm not very clear: why the sudden will change about leaving the paintings to edna instead of dale?

Edna tells Columbo something about Rudy wanting his paintings to be appreciated by “the people”. Edna would be a better caretaker of the paintings while Dale would be more selfish. But I’m not sure what motivated Rudy to this conclusion. Did Rudy have a change of his own attitude regarding how the paintings should be appreciated or did Dale do/say something that caused concern for Rudy? I don’t think the episode clarifies this.

Indeed, it doesn't, I watched it again recently, it could've gone either of the 2 ways you mentioned, to me it's very strange that he changed his mind just 10 days before being killed, so perhaps it was indeed to do with dale, a shame that there wasn't a further clarification in the movie about this.

I think it's clear from Edna's discussion with Columbo that she's been separated from Rudy for a while due in part to her infidelity. However, more recently they had become friends (again though not lovers) as they were both older and wiser.

Rudy had grown tired of the collecting, buying and selling art for the price of it and had come to realise that it wasn't the value of the art but seeing the beauty of it that mattered. Rudy and Edna were in the process of trying to decide which places such as schools, museums and galleries were worthy of having a part of the collection.

Dale simply wanted to own the collection to further his own wealth and influence in the art world. He was everything his uncle had come to dislike in the art world.