Hey @JulieTi !! Just wanted to update this idea as delivered. You can now set any company property to only accpet unique values within property settings, and use this as a unique ID within import. Instructions on how to create properties that only accept unique values can be found in this KB article. Once you create the property, you can use it in import to update existing records.
Pro Tip: you can also now choose to do an import that only updates existing records. If you choose this option (on the file upload step within import!), we’ll ignore any records that don’t match the unique ID you choose. This way you don’t have to worry about accidentally creating a new record as you’re going to update these companies.