If the item is built and then it doesn't fit, can you return or exchange it as is, or do you have to deconstruct it first?

You can return as is.

Funny you should ask that question. My son in law just purchased 2 Billy shelves and he was told if he wasn't satisfied or the item didn't work out the shelves could be returned fully assembled.

As is, that simple

It can be returned assembled.

As is

Return as is

Thank you guys very much. Highly informative