I finally got a chance to try this out myself on a place checklist using the CSV upload option (see a resulting bug report here). There is indeed a warning message for existing taxa, but the values for Description, Occurrence Status, and Establishment Means can still be updated via the Update button on each taxon, or via the batch Save button at upper right. Taxa in the batch import must be selected first, then these buttons will update existing values for the selected taxa.
Adding new species as copied text will result in the default (blank or unknown) values for each species. If adding via CSV, you can specify and save Description, Occurrence Status, and Establishment Means values for each taxon at time of upload.
All the above applies to Place checklists. I have not yet played with Project checklists.