When downloading metadata, there is a second button for configuring downloaded metadata (the gear icon right next to the download button in the Edit Metadata view).
You can choose which fields you want to download. Additionally there is an option to "append comments to existing".
But by and large, metadata download is meant to overwrite the current (assumed bad
) metadata.
When using the bulk metadata download (shortcut key is CTRL+D) you can review the metadata, which will be from the better match.
You can manually edit the new metadata (on the left) or press the <- arrow to revert a specific field to the old metadata (on the right).
I usually use the CTRL+D route, but I *thought* in Edit Metadata ==> Download you were supposed to get a choice between metadata from either source, not any kind of aggregate. I could be wrong though, as I said I don't use it enough to say with confidence what should happen.