I think you're misunderstanding something.
When you update 1 field inside of a document, EACH INDEX must look at that document and determine "do i care?"
Updating 1 document does not cause anything other than that 1 document to be processed for indexing.
Now if your question is, does modifying 1 field in the INDEX require it to scan ALL DOCUMENTS, yes, yes it does. Because you've invalidated the entire index. However that will impact only 1 index and is unrelated to documents except that it will walk over the entire document store to rebuild that 1 index.