Importing Assets from CSV file "Asset No. is already taken"

We have WHD  12.7.1 - Build #12.7.1.538

I'm attempting to import a CSV file containing asset data - updates to some entries, also some new entries.

The first line after the header contains an existing asset and we receive the error "Line 2: Asset No. is already taken..."

Do need to denote new assets differently than existing ones, or can we only import new assets