It is not possible to amend an existing Customer records via CSV.
Managing the accuracy of data is required by law, amending manually in KashFlow is slow and inefficient.
I have systems that check the company name, the registered address, checks contact details and validates and verifies VAT numbers, as it stands I have to download a copy of my customer records, process it, determine the differences, visit each customer page, amend update, rinse and repeat. This clearly doesn't happen very often, and I recognise this existing mechanism is more for migration to KashFlow than management of data already in it.
Providing a mechanism in which if a customer code already exists (maybe following a warning of some sort) that customer record should be updated. Risk of wholesale damage could be mitigated by only allowing x number of customer record updates at a time.
This has been raised before in 2017 and got a never gonna happen result.